An art teacher from one of the UK's best-known public schools last night won the BP Portrait award for a magical – if slightly unsettling – painting of his 12-year-old daughter.

Monkman said he was trying not to make a sentimentalised portrait, but to represent the changes that occur as children grow into teenagers and the problems that develop in families. His aim was to make it "a little spooky", he said.

"She is one of my biggest critics, and she did tell me I got certain things wrong," he admitted. "But although she won't come clean, I think she's quietly happy about the portrait."