Unite have been forced to admit that one of their cold-callers did encourage a Labour councillor to vote for Sadiq Khan to be Labour's mayoral candidate, despite previously strongly denying it.

Unite has been phoning London Labour members in recent weeks as part of a "consultation" process in advance of Labour's mayoral selection.

One of those who received a call was Islington councillor Paul Convery.

Convery answered on behalf of his wife and says he was subtly encouraged to consider backing Khan who the caller described as the son of a bus driver and a "decent guy".

"He mentioned [Sadiq] and then told me that he was a decent guy, and that his dad was a bus driver," he told the Evening Standard. "He started by saying 'I'm ringing you personally at the request of Len McCluskey to ask you what the issues in the London Mayoral election should be'."

A spokesperson for Unite initially released a strongly worded denial in which they claimed that any suggestion one of their cold-callers had encouraged Convery to vote for Khan was "completely untrue," "not credible" and "a lie".

However, further investigation prompted by Politics.co.uk revealed that such a call did actually take place.

"It appears that in a call to Mr Convery’s wife that the caller deviated from a script and expressed a personal opinion about 'someone called Khan'," a spokesperson admitted.

"It is clear though from the call that the caller expressed clearly that Unite has 'no preferred candidate' and that the view being expressed by the caller a personal one and not Unite’s view. Unite has reinforced to callers that they should not express a personal opinion or canvass for particular candidates.'

Convery said he still intended to give one of his votes to Khan but worried about the union's influence on the selection.

"My anxiety is that Unite could damage the integrity of the member ballot, embroil the candidate in controversy and harm its own rather bruised reputation in these matters," he told Politics.co.uk

"Unite is a Labour Party affiliate organisation and I respect and support the union's right to nominate a candidate. I support the union's intention to recruit members into the Labour party and I believe the union and its members are entitled to campaign fairly on behalf of the candidate they nominate.

"But I don't think they should be using a market research or telesales company to canvass probable candidates."

Suggestions that Unite have been "push-polling" for Khan, follow a series of behind closed-doors interviews with Labour's mayoral candidates last Saturday.

The union is officially not yet backing a candidate. However, Politics.co.uk understands that Khan has secured their support and will be endorsed by the union when it makes a formal announcement next month.

Their endorsement will follow that of former London mayor Ken Livingstone who backed Khan earlier this week.

Khan's rivals worry that the Labour party establishment are rallying around to 'stitch up' the selection process in his favour.

Last year, Politics.co.uk reported on fears among his rivals that the selection timing and processes had been arranged to increase the chances of him emerging as the candidate.

His rivals have been keen to highlight Khan's closeness to Labour's former leader Ed Miliband.

"Sadiq has been Ed Miliband's Campaign Manager, he's been very close to the leadership of the party," Tessa Jowell told LBC yesterday.

Asked if this was a criticism, she replied: "It's a description of what he's done."

Other candidates have also made similarly coded attacks.

"I don't believe that London wants a mayor that is a glove puppet of the Labour party," Diane Abbott told students at the London School of Economics last year.

Labour's selection process for mayor was originally intended to finish at the end of July but has now been delayed until September following the departure of Miliband.

Supporters of current frontrunner Tessa Jowell, believe the delay will boost Khan's chances, as many more union members will be able to sign up to take part in the primary.