Boris Johnson’s legacy as mayor of London has been savaged by one of the prime minister’s oldest friends and closest political allies in explosive evidence of the increasingly poisonous relations between the two men.

In a TV interview to be aired on Monday, David Cameron’s former director of strategy in Downing Street claims that the Conservative leadership had always been “sceptical” of Johnson standing for mayor and suggests their fears had proven to be well founded.

Steve Hilton told documentary-makers at the BBC that he could not now see what legacy Johnson would leave behind when he stands down in 2016, commenting that the Tory MP’s predecessor Ken Livingstone had more over which to boast.

Hilton, who left Downing Street in 2012 but remains close to the prime minister and the chancellor, says: “I honestly struggle to think of what his legacy is.”

The blue-on-blue attack provides evidence that the referendum debate, in which the prime minister and Johnson are on opposing sides, is set to become increasingly bitter and personal.

On Sunday, the chancellor, George Osborne, accused Johnson of playing “political games” over Britain’s future membership of the EU, while admitting the London mayor has ambitions to rise through government.

The chancellor also criticised his Conservative colleague for claiming that Britain should model itself on Canada, which has negotiated free trade with the EU but does not accept freedom of movement.

In the latest attack on Johnson, who has been playing an increasingly high profile role in promoting Britain’s exit from the European Union, Hilton takes aim at Johnson’s record in his one big political role.

In an interview filmed just last week for Boris: the London Years, which will be broadcast at 7.30pm‬ on BBC1, Hilton says: “If I recall, the idea of Boris running for Mayor of London didn’t actually come from our ranks in the Conservative party’s leadership, it actually came from the then editor of the Evening Standard who thought he would be a great standard bearer for the party in London and be a great mayor.

“That’s where the original idea came from and at the time we were still engaged in this process of trying to find someone to run and I think initially we were pretty sceptical of the idea.”

Hilton adds: “I honestly struggle to think of what his legacy is ... The Mayor of London is basically the person who runs the transport system, and that’s obviously vital, and is there to promote London in a marketing officer sense, and I think Boris has done a great job on both of those.

“But the real legacy of moving London’s transport system forward I think happened with the previous mayor, with Ken Livingstone with the big moves – the introduction of the congestion charge and the Oyster card, and in those two areas of transport and promoting London it’s really difficult to think of something specific that you could [point] at as being Boris Johnson’s legacy.”

The comments will be seen as both an attempt to undermine Johnson’s credibility ahead of the referendum poll on the 23 June, and an attempt to weaken the mayor’s leadership bid.

Hilton also admits in the interview that relations between Johnson and Osborne, who also has ambitions to replace Cameron, had been “testy” in the past.

He says: “The tension in the relationship, and not necessarily with the prime minister, but with the chancellor, as you’d expect was over funding for certain things that the mayor wanted for London; and just as anybody else negotiating on behalf of their particular interest group, there was a negotiation and sometimes that could get testy.”

During an interview on the BBC’s Andrew Marr Show on Sunday morning, Osborne was drawn on the issue of leadership. Asked about the memoirs of former Lib Dem minister David Laws, who claimed Osborne and Cameron had discussed Johnson’s leadership hopes, the chancellor said: “I don’t think it is the greatest revelation in human history to discover that Boris Johnson is interested in a job in government.”

Pushed on whether he thought Johnson wanted to be prime minister, Osborne said: “You’d have to ask him.”