A father who initially paid £4.99 for his 11-year-old daughter to use a smartphone app was shocked to discover a bill for thousands of pounds a month later.
Steve Cumming, 72, said he let her make what he thought was a one-off payment on his debit card to a firm called Roblox.
After looking at his balance a month later, he saw thousands had been charged.
Roblox says it will refund him.
Roblox is an online multiplayer game with about 100 million users worldwide.
It is especially popular with children. Its business model relies on in-app purchases. Roblox is free to download, but users can then spend money during play.
Mr Cumming wrote to BBC Radio 2's The Jeremy Vine Show to explain that his 11-year-old daughter had unwittingly run up the enormous bill while playing the game during lockdown.
"My daughter told me all her friends were playing this game and she wanted to spend £4.99," he said. "She made that purchase using my debit card on 16 April and I thought nothing more of it."