A senior Conservative Brexit campaigner has accused the Remain camp of resorting to "insults, personal attacks and petty tabloid smears", and accused Downing Street of orchestrating criticisms of Boris Johnson, as the Tory civil war over the EU referendum deepened today.

Steve Baker, the MP for Wycombe and co-chair of Conservatives for Britain, said the nature of the EU campaign so far had been "breathtakingly disheartening", singling out attacks on Mr Johnson and accusations from George Osborne that Leave campaigners are "economically illiterate".

Meanwhile, it emerged that Downing Street officials considered calling the police over the apparent leak of a letter from a senior business leader to David Cameron, with suspicion falling on Justice Secretary Michael Gove or one of his team.

The letter, sent by Serco chief executive Rupert Soames, suggested Downing Street was working with businesses to campaign for Remain before the EU renegotiation was even complete.

It is understood to have been predominately about prison services, and therefore may have been copied to the Ministry of Justice.

A Whitehall source told The Times: "It was a letter about prisons. Draw your own conclusions."

Another source told the newspaper that two senior civil servants discussed calling in the police after the contents of the letter were reported in the national press this week.

A spokesman for Mr Gove said: "The leak absolutely, definitely did not come from us. The first Michael knew about it was reading it in the papers."

However, the claim will only inflame tensions between the Remain and Leave camps.

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In an article for the ConservativeHome website, Mr Baker said that the "nastiness" of the debate must end now.

He accused Downing Street of orchestrating criticisms of Boris Johnson from Tory grandee Lord Heseltine, who said earlier this week that the former Mayor of London’s comparison of the EU’s aims to those of Adolf Hitler had "crossed the bounds of domestic debate".

"Lord Heseltine was used by Downing Street to attack Boris Johnson and convert the debate into one on personality not policy," Mr Baker alleged, adding that it was "deeply dangerous for Remain campaigners in Government to make this a debate about the future leadership of the Conservative Party."

"There have also been intolerable media smears against our leading figures and their families. It is a dark day indeed when Conservatives believe that the centre is behind such vicious briefing,” he wrote.

A Downing Street spokesman said: "