An Oxford graduate who replaced Carol Vorderman on television quiz show Countdown has been having trouble giving the correct answers to mathematical problems during filming, it has been reported.

According to the News of the World, Rachel Riley, 22, has found it hard to answer number puzzles correctly at speed.

The newspaper quoted a source as saying: "The first three weeks of filming did not go that smoothly for Rachel. It's clear she hasn't managed to match Carol's skills yet.

"If the contestants couldn't solve a problem Carol would quickly do it for them. But during the filming for her first 15 episodes the new girl made some mistakes."

Miss Riley, who has a masters degree in mathematics, beat more than 1,000 applicants to the job after Ms Vorderman quit when she was asked to take a 90% pay cut.

A Channel 4 spokesperson commented: "Rachel missed the occasional problem, but it was nerves.

"Let's get this in perspective, the girl has graduated from Oxford with a masters degree in mathematics. It's not going to be a question of ability.

"She is working on a hugely popular show, doing number tasks in front of the camera and audience - there is a lot of pressure. I think the occasional hiccup is expected."