Simon Cowell turned his "Mr Nasty" image around and paid off the mortgage of a couple whose three-year-old daughter suffers from cancer.

The multi-millionaire was touched by her story when he appeared on the US TV chat show Oprah and declared he would be her "guardian angel" from now on.

Oprah Winfrey showed Cowell a clip of Amy Stoen and her husband, Randy, from Claremont, Minnesota, who were struggling financially after their little girl Madelaine was diagnosed with an aggressive form of cancer.

The couple take it in turns to travel more than 100 miles from their home each day so the youngster, who has started losing her hair, can get treatment.

Cowell greeted the Stoens on the "Oprah" stage and told them he had written a cheque for £80,000 to pay off their mortgage.

The X Factor judge told Madelaine's parents: "I didn't know what to do in this situation other than to do something which I hope will help your situation.

"I know you are having problems with your mortgage. As of this afternoon, your mortgage has been paid off.

"It doesn't stop there," he said. "If there's any problem, I'm her guardian angel now."

Mrs Stoen hugged Cowell and he added: "I never knew doing good could feel so good."

He said he had only just realised how good it was to give and told Oprah: "It's taken me 48 years. I credit you with this."

Cowell was on the show to launch X Factor winner Leona Lewis in the US.