Portuguese police searching for Madeleine McCann today released a description of a man seen carrying a child or a large object on the night the four-year-old went missing.

The girl's parents, who welcomed the release of the description, spoke emotionally earlier of how the guilt they feel at leaving her alone "will never leave us".

At a press conference tonight, the detective leading the investigation, Chief Inspector Olegario Sousa, told journalists: "We have a suspect. He is a Caucasian man, 35 to 40 years old, medium build, 5ft 10in tall, hair mainly short, wearing a dark jacket, light or gold trousers and dark shoes.

"At around 21:30 he was walking in the area of Praia da Luz possibly carrying a child or an object that could have been taken as a child."

The inspector urged anyone with information on the man to call the police.

He refused to say whether police believed he had had any contact with Robert Murat, the Briton widely thought to have been interviewed earlier this month by detectives as an "arguido" - a formal suspect.

"This description was told to us by a witness in the case," Mr Sousa said.

"We are confident with the images we have of the child we will be able to find her. Sadly at this moment we haven't been able to reach her."

Asked why the description had not been released earlier, he replied: "If we didn't do it before it's because we didn't feel it necessary at the time."

Madeleine's parents, Kate and Gerry McCann, earlier gave their first formal interview in which Mr McCann said: "The guilt we feel having not been there at that moment will never leave us."

But Mr McCann said the chances of their daughter going missing from the safe Portuguese resort of Praia da Luz were "100 million to one".

His wife, Kate, said that at worst they were guilty of being "naive". The couple refused to criticise the police investigation, but Mr McCann did say they had expected a "British response" when they reported their daughter had vanished.

"But we're in a tiny resort. It's an on-going investigation working very closely with a lot of expert help," he said. "Many people say this is a parents' worst nightmare and it is. It's as bad as you can possibly imagine. But if all three children had been taken it will be worse than your worse nightmare," he said.

"Ultimately we need to be in control so we can help in any way possible." Mrs McCann told Sky News that they could not consider leaving the resort yet. After the first week Mr McCann said they were "devoid" of emotion but had been buoyed by support from friends, family and the wider public. "Out of an evil act has resulted so much," he said.

Earlier, it emerged British police have taken a more active role on the ground in the investigation into Madeleine's disappearance. Experts have begun working around the holiday apartment in southern Portugal where the she went missing three weeks ago. Police would not say which British force had sent the two specialists, who could be seen taking measurements and walking around with plans of the crime scene in Praia da Luz.

Eyewitnesses said they had been measuring the windows of the apartment where Madeleine and her two-year-old twin siblings, Amelie and Sean, had been left sleeping by her parents on the evening of May 3. They were also reportedly seen walking between the Ocean Club holiday complex, where the McCann family had been staying, and the nearby home of Robert Murat, the only formal suspect in the case. Mr Murat has strongly protested his innocence.

A spokeswoman for the Association of Chief Police Officers said the experts were helping on "technical matters and aspects of the investigation".

The British Child Exploitation Online Protection Centre is already using photo recognition software to analyse holiday snaps from people who were in Praia da Luz around the time Madeleine went missing. Leicester police are also running a parallel incident room that is feeding information gathered in Britain to the Portuguese police.

A spokesman for the McCanns said they would welcome extra help in the case. "The family has been saying throughout that everything that should have been done in the immediate area has been done. Part of that would be rechecking," he said. "Certainly the family will be very pleased to know that examination of the immediate area is ongoing."

Confirmation that British investigators were on the ground coincided with the arrival of two men who say they are former British soldiers and have come as freelance volunteers to carry out searches of abandoned buildings and rough land. Yorkshiremen Colin Sahlke, 46, and Steve Taylor, 32, have met Madeleine's father, Gerry McCann, in Praia da Luz. They said they were funding themselves and were interested in the missing girl's welfare rather than the £2.6m of reward money currently on offer.

A spokesman for the McCanns said the ex-soldiers were not being paid by the Find Madeleine Fund, and had been advised to coordinate anything they were doing with the Portuguese police.

Madeleine, whose case is slowly fading from media coverage, became the most prominent face of International Missing Children's Day yesterday. Figures from the National Missing Person's Helpline show that between May 3 and May 15, 450 young people were reported missing in the UK. Home Office estimates show 210,000 people reported missing in Britain each year, with up to two-thirds of them under the age of 18. Initiatives across Europe under consideration include a plan by the European commission for a single telephone number - likely to be 116 - on which to report missing or sexually exploited children.