Bill Bush, head of BBC Analysis and Research and former chief of staff to Labour left-winger and London mayoral hopeful Mr Livingstone, is to take up his new post in the near future.

His new position is head of research in 10 Downing Street. He cleared his desk at the BBC when his appointment was announced on Monday, though his exact starting date was still being negotiated.

Mr Bush worked for the GLC between 1975 and 1986, latterly as Mr Livingstone's closest aide. He joined the BBC in 1990 as head of political research.

Ken Livingstone: "Tony Blair is very, very lucky to have him"

His job with Downing Street is a civil service, as opposed to political, appointment, and has been specially created. It will include briefing the prime minister and other senior Downing Street staff on all issues.

He will also assist in prime ministerial speeches, and provide advice for debates and Mr Blair's media interviews.

'Nothing to do with London mayor'

Speaking to BBC News Online, Mr Bush denied that the wide-ranging remit of his new position would inevitably include helping Downing Street deal with Mr Livingstone's campaign to become Labour's candidate for London mayor.

"Obviously, to an extent the bureaucracy of my old job with the GLC and the position at Downing Street is similar," Mr Bush said.

"But it's nothing to do with the mayor for London stuff at all. It is not remotely connected to all that."

The Labour leadership is anxious to keep Mr Livingstone's name off the ballot paper for the internal selection contest to choose the party's mayoral candidate.

Opinion polls both inside and outside Labour have consistently shown the Brent East MP as favourite to become London's first directly elected mayor next year.

Attempts by party managers to come up with an alternative candidate capable of at least matching Mr Livingstone's popularity ratings have so far failed.

Bill Bush told BBC News Online he had not spoken to his old boss about his new job - and that he could not guess what his reaction might be: "I've no idea what he'll think. But I know Ken has a great sense of humour."

Mr Livingstone told BBC News Online he was pleased for his former charge. "I'm delighted for him, and Mr Blair is very, very lucky to have him," he said.

He added that any impact Mr Bush's new activities might have on his own chances of becoming Labour's mayoral candidate could only be positive. "I think it's a very good sign," Mr Livingstone said.

"Bill Bush used to work for me. And if he can work with the government and Downing Street, it's a good sign that I can, too."

The prime minister's official spokesman said his new colleague "will be building a proper research database. It is about knowing how policies impact and how they are being delivered".