The 19-year-old model, who took over from Denise Van Outen in February, is said to have handed in her resignation to both Channel 4 and the programme's producers Planet 24.

A Channel 4 spokeswoman said: "We are grateful to Kelly for all her hard work over the past five months and wish her every success in her future career."

It follows speculation in the press earlier in the week about her future on Channel 4's early morning show as she took a two-week holiday with actor boyfriend Jason Statham.

Brock was due to return to work on Monday. In her absence presenters Liza Tarbuck and Sara Cox had filled in for a week each.

Sara Cox has been standing in for Kelly Brook

Their strong performances prompted suggestions that programme bosses might end her contract before it ran out at the end of the year.

Both Cox and Tarbuck are now considered hot favourites to take over full-time. The Channel 4 spokeswoman said: "Sara Cox will be carrying on for the next week but beyond that we cannot say."

Brook, who was best known for her modelling work for Triumph bras, was a surprise choice with little TV experience when appointed late last year.

She beat the stand-in presenter Melanie Sykes and a raft of more experienced presenters, including Liza Tarbuck, to the coveted position.

But she failed to recreate the powerful chemistry which Vaughan had with Van Outen.

She also came under pressure after allegedly stumbling over words and the programme's assistant producer had to request scriptwriters to keep difficult words to a minimum.

Rise to fame

Her first break came at the age of 15 when she won a modelling competition on a local newspaper.

Denise Van Outen: A hard act to follow

She went on to become the model for Triumph bras, as well as doing a low-key presenting job on the Granada TV fashion show VPL.

However, she emerged as the favourite to replace Van Outen after proving to be a hit with focus groups, and co-presenter Vaughan.

"I felt most comfortable and relaxed with Kelly," he said after her appointment. He added:"She was the one who was most herself and for 10 hours of TV every week you can't pretend to be something you're not."

Brook had been in the running for Channel 5's revival of It's A Knockout. But the TV station is now thought to be focusing on three other candidates.