The hosts of BBC children's programme Blue Peter have apologised to viewers after the results of a competition were faked last November.
A technical problem meant viewers calling for the Whose Shoes contest did not get through to the studio.
Instead, a visiting child was asked to pose as a caller, and won the prize.
Host Konnie Huq said: "We'd like to say sorry to you because when this mistake happened we let you down." The BBC did not profit from the calls.
BBC Children's controller Richard Deverell called the incident a "serious error of judgement".
The contest, on 27 November, was raising money for children orphaned by Aids in Malawi.
Callers were asked to phone in and identify a mystery celebrity's shoes. More than 13,800 people entered, with calls costing 10p each, including 3.25p for the Unicef charity.
But an "unavoidable technical difficulty" meant producers were unable to access the callers' details and so found a stand-in. The winner could select a prize from a list of children's toys.
The child they found was already in the studio because she had won a separate competition.
Premium phone line regulator Icstis is investigating the incident, which was discovered when another visitor to the programme set, Mona Zahoor, wrote to the BBC's Have Your Say messageboard.
"We were all quite horrified when it happened," she told the BBC News website.
An internal investigation confirmed the error and a separate independent review will be carried out into the circumstances surrounding the competition.
"This edition of the programme fell short of the high standards Blue Peter viewers quite rightly expect," Blue Peter editor Richard Marson said.
"We are very sorry for the way this competition was conducted."
The competition has been re-run, with a new winner chosen from the original callers. The problem will "never happen again", viewers have been told.
Blue Peter will celebrate its 50th birthday in October 2008
"I would like to apologise unequivocally to viewers, to all the children who took part in the competition," Mr Deverell said.
"We have already apologised directly to the child involved and her family for this incident."
Mr Deverell called the faking of the competition an "exceptional incident", and said the person responsible had acted "in a panic".
But speaking to Radio Five Live, he refused to rule out the possibility that staff would be sacked over the incident.
A freephone number - 08000 565 363 - is available for viewers who took part in the competition and would like more information.
Viewers can also find information on the Blue Peter website.
But Ms Zahoor, whose information led to the discovery, said she thought the BBC's reaction was "silly".
"I didn't realise that it would be blown out of all proportion," she said, adding that she had refused to lodge a formal complaint about the show.