Fears were growing last night that two ten-year-old girls missing since Sunday have been abducted as police released a poignant last picture of the youngsters.

The photograph of Holly Wells and Jessica Chapman posing in their Manchester United shirts was taken only 90 minutes before they vanished.

Detective Superintendent David Hankins outlined his fears as a growing search involving more than 250 police failed to find any trace of the youngsters.

"From the outset, we thought this may have been a prank," said Mr Hankins.

"And then we thought there may have been an accident. Now we are having to consider the possibility of abduction."

When the girls were last seen near their home in Soham, Cambridgeshire, at about 6.30pm on Sunday, they were dressed in matching Manchester United tops, with the name Beckham and the number 7 on the back of each, dark trousers and white training shoes.

The photograph released yesterday was taken at Holly's house, where they had spent much of the day playing. A clock on the wall shows the time to be just after 5pm.

The girls' families were given a glimmer of hope on Tuesday when a woman said she had seen two girls matching Holly and Jessica's description walking outside her house and looking playful and happy.

The woman lived at Little Thetford, Cambridgeshire, beside the main road between Ely and Cambridge - about eight miles from Soham.

But hopes that motorists travelling on the road would call police to confirm the sighting - at 6.45 am on Monday - were dashed yesterday.

Police said they had received no calls which corroborated the woman's sighting and they were coming to the conclusion that the girls seen were not Jessica and Holly.

Specialist search teams were yesterday continuing to comb fields, hedgerows, outbuildings and waterways in and around Soham.

Two senior detectives from Sussex Police who worked on the case of Sarah Payne also joined the investigation team to offer their assistance and advice. But the officer leading the search team, Superintendent Simon Edens, said last night that no clues to the whereabouts of Jessica and Holly had yet been found.

It was revealed that detectives had seized a white van, on which they were carrying out forensic tests, which was reported to have been cruising in Soham on the night Holly and Jessica vanished.

But senior officers said they were not classing the seizure of the van as a significant development in the search.

The girls' parents, Sharon and Leslie Chapman and Kevin and Nicola Wells, again broke down at a media conference where they were making fresh appeals for help in finding the children.

Mrs Chapman urged anyone who might be holding the youngsters: "Give them back - just give them back."

Mrs Wells said: "Everyone with children must know what we are going through."

Detectives are now considering the possibility of staging a reconstruction of the girls' last known movements.

* Last night, Express Newspapers offered a £1m reward for information leading to the conviction of anyone found responsible for the disappearance of the girls.

The Sun and The News of the World have put up a joint reward of £150,000 for anyone solving the mystery.