Boris Johnson has suggested that allegations about his private life and previous conduct have been made because of attempts by some people to "frustrate" Brexit.
The Prime Minister accepted it was "inevitable" that he would come under "shot and shell", but said he thought it was "raining down" on the Government because of his vow to take the UK out of the EU on October 31.
Claims that Mr Johnson squeezed the thigh of journalist Charlotte Edwardes at a private lunch at The Spectator magazine's HQ shortly after he became editor in 1999 have overshadowed the Conservative Party conference in Manchester.
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He has also faced allegations about his relationship with American entrepreneur Jennifer Arcuri and whether she enjoyed preferential treatment while he was mayor of London.
Mr Johnson insisted Ms Edwardes' allegations are "not true" and said it was "very sad" that someone should make such claims.
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But in an interview with LBC radio, the PM suggested the stories had come to light at this time because of his Government's stance on Brexit.
"I think there's a very good reason, and that is that I've been tasked one way or the other to get Brexit done by October 31.
"And there are quite a lot of well-meaning and highly intelligent people who basically think that that would be something they don't want to see, and I think that there is a concerted effort now to frustrate Brexit."
Asked if the stories were politically motivated, he replied: "I don't want to impugn people's motives or to minimise the importance of the issue. All I can say is that allegation is certainly not true.
"And I think, generally - you asked me about why is all this shot and shell raining down on the Government - I think it is because we're going to get on and deliver Brexit by October 31."
Meanwhile at the conference on Tuesday, the Justice Secretary will outline plans to keep sexual and violent offenders behind bars for longer.
Robert Buckland will confirm he will end the system which sees some of the most serious offenders released after serving just half their sentence.
The move - and a warning to criminals from Home Secretary Priti Patel that "we are coming after you" - is the latest sign of the Tories seeking to burnish their credentials as the party of law and order ahead of an expected election.
In her conference speech, Ms Patel will say: "As we renew our place as the party of law and order in Britain, let the message go out from this hall today: To the British people - we hear you.
"To the police service: We back you. And to the criminals, I simply say this: We are coming after you."
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