The eager 25-year-old was scheduled to take the helm of the flagship show on Monday, but decided to grab the microphone on Friday instead.

Best friends: Zoe Ball and Sara Cox

Cox told listeners she had locked DJ Scott Mills - who has been hosting the show since Zoe Ball quit three weeks ago - in a cupboard before taking over the programme.

A Radio 1 spokesman said: "Sara has been rehearsing the show for weeks and was keen to start early."

Cox, who has been practising for her debut for days, launched the show at 6.30am by playing the Robbie Williams single Let Me Entertain You.

She said: "It's so exciting. It's a bit premature, we've hijacked the studios."

During her two-and-a-half-hour show, Cox held a listener phone-in on initiation ceremonies and joked: "I've already flushed my own head down the loo to get it over with."

The former model rose to fame as a presenter on Channel 4's risque Girlie Show, then joined the British arm of music station MTV. She has recently had a regular spot on Channel 4's Big Breakfast.

Like her best friend Ball, she is linked to a pop star, dancer Leeroy Thornhill from the Prodigy, whom she is due to marry next year.