Have the McCanns really chosen the best private detectives to find Madeleine?

The heavy oak front door swings open and a dog sidles through the gap, sniffing at the marble stair outside.

It is not a bloodhound, as one might expect to find at the headquarters of what is currently the world's most high-profile and - allegedly - tenacious private detective agency, but a black poodle called Royale.

A woman is calling it as I step past and inside. She stares at me, puzzled, then scuttles off, leaving me alone in the art nouveau entrance hall, which smells of cigars and stale perfume.

Silence reigns. No buzzing telephones. No banks of computers. No staff obviously waiting for, or working towards, the breakthrough - even though they are "very, very close to finding those responsible", according to the boss.

At one end of the office suite, overlooking the street, is a dark and empty room with huge, buttoned leather armchairs.

Francisco Marco of detective agency Metodo 3

An old man, with a mournful face and a grey three-piece suit to match, is standing in the shadows watching me. He cannot speak English, it seems.

A large number of box files are piled on a side table, at least suggesting some recent activity.

It is only then, above the paperwork, that I see it, the first evidence of the reason for my visit. It is a small poster bearing the word 'Missing'.

Below that is the now iconic face of four-year-old Madeleine McCann, who disappeared from her family's holiday apartment in Praia da Luz, Portugal, seven months ago.

Occupying the first floor of an elegant block in central Barcelona, this is the office of the Metodo 3 detective agency.

It claims to be the biggest in Spain. Now they are arguably the most famous on the planet, having been hired in early autumn by Madeleine's parents, Kate and Gerry McCann.

Outside the realms of fiction, private eyes rarely have any kind of public profile. But the McCann case is different.

Metodo 3, and in particular its director Francisco Marco Fernandez, is generating the kind of publicity enjoyed by sleuths such as Sherlock Holmes or Hercule Poirot.

"Our staff interviewed the McCanns for ten hours, enough time for us to tell if they were trying to fool us," he told a Spanish newspaper on taking the case.

"My specialists assure me they are not hiding anything. I would not risk the prestige this agency has gained over 23 years without being convinced there is a case. For me this is a special case."

That is something of an understatement. With 40 men and women allegedly on the trail between the Atlantic coast and Saudi Arabia, Metodo 3 is being paid £50,000 a month, earning them £300,000 over the six-month contract.

The bill is being picked up by the Find Madeleine Fund, which has so far raised £1.1m. Some £700,000 has already been spent on the inquiry and publicising Maddie's disappearance.

The Madeleine McCann case has shown the Portuguese police to be a laughing stock. But is Metodo 3 any more capable of solving the case?

You might ask why the McCanns chose a Spanish agency 700 miles away from the scene of the crime. One reason is that it is illegal in Portugal for a private investigation to be carried out on a case that is being pursued by the police.

A jail sentence of two years can be imposed for what could be construed as an obstruction of justice.

Off the record, the police say they will tolerate Metodo 3 - as long as they don't interfere with evidence or speak to witnesses. Some obstacle.

Nor have doubts over the private sleuths' abilities been lessened by their repeated extravagant, if not reckless, pronouncements.

The latest inflated claim was made last month to a BBC Panorama team, when Francisco Marco, son of the firm's founder, proclaimed: "We're 100 per cent sure Maddie is alive."

He added: "We're sure she was abducted and we are very, very close to finding those responsible."

The claim caused an embarrassed Clarence Mitchell, the McCann's official spokesman, to brief the media that the private detective had simply got "carried away".

They were not nearly as close to a breakthrough as had been depicted.

The question has now become one of the agency's credibility.

"With an average of 2,000 cases per year, we have established ourselves as the Number One company in the Spanish market," boasts Metodo 3 on its website.

It lists its areas of expertise as being: insurance, financial, legal, franchises, fraud, mutual insurance, patent and trademark falsification, information protection and due diligence.

However, there is no suggestion of missing persons, or even cases outside the commercial sphere. Indeed, in a 2003 interview, Francisco Marco said:

"As we're not specialists in dealing with private individuals, we stayed away from this area.

"Our focus is companies. Many detective agencies deal with private matters such as infidelities but we want to make this for business people and businesses... Since detective work is mostly in the private field (family, infidelity, etc) we decided that our target was the business area. And there is where we have been ever since."

It emerged last week that five senior members of the company were once arrested in a phonetapping-scandal linked to leading politicians and businessmen.

Their offices were raided and police seized guns, ammunition, listening equipment, cassettes and transcripts of taped phone calls.

But the 1995 case was dropped by a judge after defence lawyers levelled accusations of police entrapment.

Manuel Marlaska, a journalist from Spain's best-known investigative magazine Interviu, says: "They are the most prestigious detective agency in Spain.

"But the work they are doing now seems strange. They do not have any experience of working with such a high-profile case as that of Madeleine McCann.

"Most of their work is to do with investigating company fraud."

This is perhaps reflected in the fact that all of Metodo 3's senior directors are lawyers rather than former policemen.

One well known missing persons case that the firm has been linked with is that of Francisco Paesa, an arms dealer and double agent.

Working with the Spanish government in the 1980s, Paesa had sold missiles to Eta, the Basque terrorist group. The weapons were secretly fitted with surveillance devices and, as a result, important arrests were made.

Paesa also helped expose Luis Roldan, the corrupt former head of the Spanish Guardia Civil. The arms dealer then faked his own death in Thailand and his family lodged a death certificate in a Spanish court.

The authorities were not convinced that he was dead, suspecting that their double agent had disappeared with the money stolen by Roldan.

But Paesa was tracked down by Metodo 3 - acting on behalf of a client who claimed to have been defrauded by him - in Luxembourg, living under the name of Francisco Pando.

While they got their man, this was more of a fraud case with added glamour, not a possible child murder or abduction involving paedophiles.

Despite the fact that no reference to them appears on its website, Metodo 3 has claimed involvement in 23 cases of missing children.

The firm boasted that it had cracked them all, including the rescue of a teenage boy from the clutches of an "evil pervert".

But these claims have never been substantiated and without more detail are impossible to verify. Spanish police say they can neither confirm nor deny the claims.

What then of Metodo 3's performance so far in the McCann case?

In October, its boss Francisco Marco declared he was 'convinced' Madeleine had been "kidnapped to order" by a Moroccan paedophile gang and spirited out of the country. He now knew she was being held in the Rif mountains.

This sparked a rash of sightings of blonde girls in Morocco, to which Metodo 3 agents were sent.

Morocco's interior ministry eventually felt moved to declare that Metodo 3's reports had no credibility and the agency's theories were also questioned by a prominent British crime profiler.

Metodo 3's insistence that Madeleine was abducted by a paedophile gang who stole the child to order showed a "distinct lack of understanding of paedophiles and how they work", criminal analyst Mark Williams-Thomas warned.

Mr Williams-Thomas, who worked on the Sarah Payne murder inquiry, said: "If Madeleine was abducted by a predatory paedophile the likelihood of her surviving after 48 hours are slim.

"But if she is alive she would not be out in the open and spotted by general members of the public. She would be hidden away. Even in these remote areas of Morocco, nobody could risk her being seen."

There was one further question: why on earth would Metodo 3 announce a possible lead before it had been thoroughly checked out?

In fact, why was Francisco Marco apparently showing his hand at every turn?

He has gone on the record as being 'certain' that Madeleine was seen on May 4 in northern Portugal.

Recently Metodo 3 were briefing the media that they believed Madeleine had been abducted by British expat suspect Robert Murat, his German girlfriend Michaela Walczuch and her estranged husband Luis Antonio.

Ms Walczuch says she has an alibi. In this tangled mass of claims and contradictions the private detectives are beginning to resemble the discredited local police they were brought in to replace.

However, Metodo 3 has also clearly got under the skin of the Portuguese police. On the day that I visited the firm in Barcelona, a police source described the Spanish agency as 'small fry' adding 'they are irrelevant'.

When I visit, the Metodo 3 nerve centre could not be more low key.

After whispers in a back room a thirtysomething man eventually appears and tells me that no, the agency will not be responding to police brickbats. In fact, there will be no statements at all: "We are not attending the press today."

I am politely ushered out, as Royale, the disobedient poodle, is finally collared and persuaded to return. The ace manhunters have at least got their dog.