'We would be better off with Cameron': Union chief brands Miliband a 'smug and arrogant s***'

One of Labour's union paymasters condemned him as 'smug' and an 'arrogant s***', and warned that the party would be 'better off with Cameron'.

Details of the expletive-laden rant were disclosed by shocked union sources at the TUC conference in Brighton.

Attack: Union boss Derek Simpson and Foreign Secretary David Miliband

Derek Simpson's vitriolic tirade revealed the extent of the unions' opposition to Mr Miliband as a possible successor to Gordon Brown if there is a contest.

They fear the former adviser to Tony Blair would take on the unions by forcing through another round of private sector solutions for reforming the public services.

Mr Simpson, joint head of the powerful Unite union, accused Mr Miliband of wanting to take Britain back 'to the failings of Blairism'.

Last night Labour MPs feared Mr Simpson was doing the dirty work for Mr Brown because his union employs Charlie Whelan as its national political director.

Mr Whelan was Mr Brown's spin doctor in opposition and in his early years at the Treasury, and has remained close to his old boss.

After union officials confirmed that Mr Simpson's attack had been deliberate, Labour sources speculated that it had been engineered by Mr Whelan.

Mr Miliband enraged Downing Street this summer by staging a blatant attempt to position himself as an alternative leader if Mr Brown is ousted.

He wrote an article for the Guardian newspaper in which he failed to refer to Mr Brown by name, and then pointedly refused to rule himself out as a challenger.

It sparked a furious backlash from the Prime Minister's allies that fuelled the simmering civil war behind the scenes.

In an interview with the Observer ahead of this week's TUC conference, Mr Simpson said: 'We might as well elect Cameron. We might be better off with Cameron.

'Why should we elect a young fresh face when we have already got one in Cameron with policies that are not dissimilar?'

He urged Mr Brown to tell those who support Mr Miliband, including Charles Clarke who called on Mr Brown last week to stand down 'with honour' to 'sod off'.

Former Foreign Office minister Denis MacShane rallied to Mr Miliband's side last night and called for an end to the 'screeching abuse' inside the party.

'For a senior union leader to use bar-room language to abuse the Foreign Secretary can only be a gift to the Conservatives.

'There are people behind the scenes who think that by screeching abuse at senior Labour figures like the Foreign Secretary or indeed the Prime Minister their cause will be advanced.

'The only beneficiaries are the Conservatives. I wish they would grow up.'

Mr Miliband refused to be drawn into the fray last night.

A spokesman said: 'This is a matter for Derek Simpson. We are not going to say anything.'

Mr Simpson tried to play it down by saying the issue was not about individuals. He claimed it was 'regrettable' that he had been quoted out of context.

In his interview he said there was no-one in the Labour Party - with the exception of left-wing backbencher Jon Cruddas - who could do a better job than Mr Brown.

But he warned it was vital that Mr Brown stop 'pandering to Tories inside the Labour Party'.