PM is urged to end his feud with Foreign Secretary Miliband

Labour MPs and diplomats warned that the pair’s tense relationship has affected the Government’s speed of response to the Georgia crisis and threatens to overshadow the Prime Minister’s high-profile trip to China this week.

Face-off: David Miliband (Left) and Gordon Brown are 'barely speaking'

The warnings came as Mr Brown’s advisers put the finishing touches to a ‘September fightback’ plan designed to avert a leadership challenge by Mr Miliband, who made clear his ambitions in a newspaper article last month.

But Mr Brown’s hopes of recovering his authority over the party have been dealt a series of blows, including:

 - A senior Scottish Nationalist politician telling The Mail on Sunday that the party was considering breaching convention by forcing Gordon Brown to call an immediate by-election in the Glenrothes seat left vacant by last week’s death of Labour’s John MacDougall. The likely loss of the seat could prove a tipping point for the Brown leadership.

 - Claims that a key member of ‘Team Miliband’ has advised political broadcasters ‘not to take a holiday’ in September.

 - News that unions are anticipating a leadership contest by devising strategies to enable Health Secretary Alan Johnson to block Mr Miliband.

 The Glenrothes by-election, where the SNP are favourites, can be held any time between September 11 and December 4. Mr Brown is almost certain to want to delay it until after the Labour conference at the end of next month.

But it is only a tradition that a deceased MP’s party decides when the poll is called and last night one senior SNP figure was considering calling the by-election immediately.

Mr Brown, who returned to Downing Street this weekend after a three-week break, was deftly out-manoeuvred on Georgia by Tory leader David Cameron, who was quick to speak out in support of the country and was rewarded with an invitation to the capital Tbilisi yesterday.

Foreign Office officials say part of the reason for Labour’s inaction is that Mr Miliband is ‘barely speaking’ to Mr Brown.

Last night, Labour MP Graham Stringer called for a quick contest between the two. ‘Unless this issue is settled, the country will suffer in the most important respects – diplomacy and international relations,’ he said.

And senior Labour MP Fabian Hamilton said: ‘Feuding does not interest people. Divided parties do not get elected.’

On Mr Brown’s return from China, he will put the finishing touches to an expected Cabinet reshuffle and an economic recovery package. But defeat in an early Glenrothes by-election could blow it out of the water.

Phil Collins, the political adviser seen as the mastermind behind the Miliband campaign, recently said that the PM needed a ‘magician’ to save his job and that ‘a monkey with a typewriter’ could improve his speeches.

Both the BBC and Channel 4  are compiling Miliband specials to be used in the event of a successful putsch, and one insider said Mr Collins advised them not to take a break next month.

A spokeswoman for Mr Miliband denied that Mr Collins was playing any role in a putative leadership campaign.

‘There is no campaign,’ she said. Mr Collins did not return calls.

But the most powerful trade unions, including Unite, are understood to want Mr Johnson and would try to block Mr Miliband by using their voting muscle to refuse to endorse a leadership contest.