Police are investigating threats to kill BBC director-general Tony Hall, reportedly linked to his decision to axe Top Gear presenter Jeremy Clarkson.

Scotland Yard confirmed it was looking into allegations made on Wednesday, when Hall confirmed he would not be renewing Clarkson's contract because of his unprovoked attack on producer Oisin Tymon at a North Yorkshire hotel.

It emerged yesterday that the director-general - officially crossbench peer Lord Hall of Birkenhead - and wife Cynthia had been under 24-hour guard since the threat was received.

A Scotland Yard spokesman said: "Police in Westminster are investigating an allegation of threats to kill. The allegation was reported to police on Wednesday March 25. Inquiries continue. No arrests have been made."

Images published showed security guards on patrol outside Mr Hall's Oxfordshire home. It was suggested the cost of the round the clock security was around £1,000-per-day to protect the BBC's top executive. It also said that the BBC's head of security had asked for ex-special forces soldiers to join the team.

Mr Hall, former chief executive of the Royal Opera House, took over the £450,000 BBC post in April 2013 to replace George Entwistle, who left the corporation in the wake of the Jimmy Savile sex abuse scandal.

On Wednesday Mr Hall announced that Clarkson, a popular but divisive figure during his time at the hugely successful BBC Two motoring show, would not be retained, saying ''a line has been crossed'' and that ''there cannot be one rule for one and one rule for another''.

The inquiry into the fracas was led by BBC Scotland Director Ken MacQuarrie.

Mr Tymon had his lip split by Clarkson in a 30-second-long assault on March 4 and took himself to hospital with his injuries. He was also shouted at by the former Top Gear presenter in a torrent of verbal abuse.

Clarkson reported the incident to the BBC five days later and was suspended by the broadcaster on March 10.

A BBC spokeswoman said: "We wouldn't comment on security matters."

Meanwhile, BBC Radio 2 presenter and well known TV personality Chris Evans has again ruled himself out of the running to replace Clarkson.

Evans is a well known car enthusiast and collector of vintage vehicles, including classic Ferrari models.

In his regular newspaper column, Evans suggested that the programme would be able to survive the loss of its outspoken star.

He said: "As for Jeremy no longer being on the show: sure it's a huge loss. Top Gear was the show he was born to do. But by no means is it a fatal blow."

Politics, music and football all survived the departures of Winston Churchill, John Lennon and George Best, Evans said.

Reiterating he would not be taking part in the show, the One Show presenter said: "I love Top Gear. But I never want to watch me on it. Of course, I'm flattered that others seem to think my involvement would be a good idea. But they're hugely misguided.

"I also happen to think I actually wouldn't be very good at it."

Last week Evans told listeners to his Radio 2 show "not only [are rumours he'd replace Clarkson] not true, it's absolute nonsense", adding that it was "never going to happen".

TV presenter Jodie Kidd, who is another car enthusiast, is among the favourites along with Dermot O'Leary who announced on Friday that he has left his role as the compere on the X Factor. Kidd tried to avoid answering questions about being linked to the show when she appeared on a cookery show on the BBC on Saturday.