The competition had been raising money for children's charity Unicef
The BBC has been fined £50,000 after the results of a Blue Peter competition were faked last November.
The show allowed a child visiting the studio to pose as a caller when technical problems stopped real calls getting through to the studio.
Media watchdog Ofcom criticised the BBC for "negligence" and for "making a child complicit" in the deception.
It is the first time Ofcom has imposed a financial penalty on the BBC for an editorial failure.
It fined the BBC £45,000 for faking the competition and £5,000 for repeating the show on children's channel CBBC.
"We regret that Ofcom found it necessary to impose a fine," the broadcaster said.
"As our previous statements have made clear, we fully accept the seriousness of this case and apologise for the breach of trust with our audiences."
A spokeswoman confirmed that the fine would be paid out of licence-fee money.
The contest, on 27 November, was raising money for children orphaned by Aids in Malawi.
More than 13,800 people entered, with calls costing 10p each, including 3.25p for the Unicef charity.
But an "unavoidable technical difficulty" meant callers' details could not be accessed, so a researcher asked a child visiting the studio to stand in.
"The decision to involve the child in the deception for the sake of expediency demonstrated a casual lack of regard for the welfare of that child," Ofcom said in its ruling.
It added that the incident had only occurred because of a "background of management and compliance failures".
These included inadequate training for the researcher running the competition, who made the decision to select a fake "winner" without getting approval from senior members of the team.
Ofcom also noted that the incident was only discovered three months later when another visitor to the programme set, Mona Zahoor, wrote to the BBC's Have Your Say messageboard.
"Had no complaint actually been received from a member of the public, knowledge of what had happened would almost certainly not have been communicated beyond the Blue Peter team," Ofcom said.
The Blue Peter team apologised after the error was discovered
It added that actions taken by the BBC to rectify the mistake - including an on-air apology, a re-run of the competition and the offer of a refund to participants - would not have happened if Ms Zahoor had not contacted the corporation.
The decision not to notify senior managers outside the Blue Peter team of the incident was a "serious editorial failure", it added.
A repeat edition of the programme on CBBC later the same day also incurred a fine because it did not make it clear that the competition lines were closed.
More than 3,500 further calls were received during the repeat. The BBC accepted its on-screen captions "were not adequate".
However, Ofcom agreed with the BBC that Blue Peter had not set out to mislead its audience on either occasion, and noted that both BBC One and CBBC had a good compliance record in the past.
It also took into account that the fine would be paid from the licence fee, and set the amount accordingly.
After the incident came to light in March, Blue Peter host Konnie Huq told viewers: "We'd like to say sorry to you because when this mistake happened, we let you down."
The programme's editor, Richard Marson, has since left his job and been given a role elsewhere in the BBC.
Responding to Ofcom's decision, the BBC Trust - which oversees the corporation's activities - said it expected bosses to "learn from these breaches".
"The culture of the BBC must be such that any proposal to mislead audiences is instantly dismissed as wholly inappropriate," it said.
The Trust, which represents the interests of licence fee-payers, has already commissioned a review of the use of premium rate phone-ins on BBC programmes.