A leading Conservative EU exit campaigner has criticised "petty tabloid smears" by the Remain campaign and suggested Downing Street is behind "vicious briefings" against his side.
MP Steve Baker called for an end to "personal nastiness" warning "deep divisions" in the party were emerging.
Both campaigns have traded barbs - with Boris Johnson at odds with David Cameron and Lord Heseltine this week.
A Downing Street spokesman said "we don't accept" Mr Baker's claims.
In other EU referendum campaign developments:
- Vote Leave's Lord Owen delivers a speech urging people to support an EU exit on 23 June
- Former New York mayor Michael Bloomberg has come out in support of a Remain vote
- Treasury minister Greg Hands gives evidence to the Treasury Committee's inquiry into the costs and benefits of EU membership
- Credit agency Moody's warns a Brexit would hit the European economy and have a "significant impact on confidence"
The government is campaigning for a vote to remain in the EU in the 23 June referendum and Prime Minister David Cameron, whose party is divided on the issue, has previously called for a "respectful" debate.
But in an article for the ConservativeHome website, Mr Baker, the co-chairman of Conservatives for Britain, said he had found the recent debate "breathtakingly disheartening".
Former London mayor Mr Johnson also sparked a row by referring to Barack Obama's "part-Kenyan" ancestry after the US president backed a vote to remain, and described international figures backing the government's case as appearing in "Downing Street hostage videos".
Mr Baker pinned the blame on the Remain campaign - which he said had descended "into insults, personal attacks and petty tabloid smears on key people".
He criticised Chancellor George Osborne's dismissal of the Brexit case as "economically illiterate" and cited the example of Lord Heseltine, who told the BBC he would be "very surprised" if Mr Johnson became prime minister after his "preposterous, obscene" remarks.
"I was appalled that such a distinguished figure as Lord Heseltine was used by Downing Street to attack Boris Johnson and convert the debate into one on personality not policy," said Mr Baker.
Another leading Leave campaigner, former cabinet minister Iain Duncan Smith, dismissed Lord Heseltine as "from the past" following his intervention.
Mr Baker claimed there had been "intolerable media smears" against leading Leave figures and their families, adding: "It is a dark day indeed when Conservatives believe that the centre is behind such vicious briefing."
He added: "If we're to come together after this referendum, personal nastiness must end now."
'Dagger in heart'
Speaking on BBC Radio 4's Today programme he said the Remain campaign "can't make an honest and positive case" because they "know it's a losing argument".
"So we've been reduced to fear, uncertainty and doubt on the Remain side, and lashing out at individuals," he said.
He added: "What essentially I'm saying is Queensberry Rules - so a full-frontal assault with due warning is fine - but the dagger in the heart inserted from the back through whispering in dark corridors is not OK."
A Downing Street spokesman said: "We don't accept Steve Baker's article.
"All our arguments are rooted in the thought that we are stronger, safer and better off in the EU."
The spokesman expressed confidence that the Conservative Party would be able to unite after the referendum.
Health Secretary Jeremy Hunt also said Mr Baker was wrong, and added: "Arguments have been made forcefully on both sides."
In other EU referendum news, former New York mayor Michael Bloomberg said a UK vote to leave the EU could hit UK-EU trade and leave British-based "employees worse off".
Writing in The Times, he warned financial jobs could leave London for Paris or Frankfurt and said "a vote to leave is a risk".