Boris Johnson has dodged questions over whether he has used cocaine, despite previous admissions that he had taken the drug when he was at university.

The leadership frontrunner refused to answer questions on his conflicting statements over his use of the class A drug, when he launched his bid to be the next Tory prime minister.

Conservative leadership hopefuls are facing scrutiny over their consumption of illegal drugs after Michael Gove campaign was nearly derailed by the revelation that he had used cocaine more than 20 years ago, when he was working as a journalist.

Mr Johnson previously admitted using cocaine as a 19-year-old but then also claimed he had not actually snorted the drug in a separate interview.

Pressed directly on whether he had ever taken cocaine, Mr Johnson replied: “I think the canonical account of this event when I was 19 has appeared many times and I think what most people in this country really want us to focus on is what we can do for them and what our plans are for this great country of ours.”

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Asked whether he had ever done anything illegal, Mr Johnson said: “I cannot swear that I have always observed a top speed limit of 70mph.”

The row centres on an appearance he made on the BBC’s Have I Got News For You in 2005, Mr Johnson when he admitted being given the drug but said he had not actually taken it.

He joked: “I think I was once given cocaine but I sneezed and so it did not go up my nose.

“In fact, I may have been doing icing sugar.”

But in an interview with Piers Morgan in GQ in 2007, Mr Johnson said: “Yes. I tried it at university and I remember it vividly.

“And it achieved no pharmacological, psycho-tropical or any other effect on me whatsoever.”

Pressed on whether the drug actually went up his nose, he said: “It must have done, yes, but it didn’t do much for me, I can tell you.”

Mr Johnson’s answers are unlikely to quell the interest in his past, which could prove a problem during the leadership race.

His leadership launch was dominated by questions over his previous gaffes, including his claim that Muslim women wearing burkas “look like letter boxes”.

Challenged on the point, Mr Johnson said: “Occasionally some plaster comes off the ceiling as a result of a phrase I may have used, or the way that phrase has been wrenched out of context by those who wish for reasons of their own to caricature.

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“But I think it’s vital for us as politicians to remember that one of the reasons that the public feels alienated now from us all as a breed, is because too often they feel that we are muffling and veiling our language, not speaking as we find – covering everything up in bureaucratic platitudes, when what they want to hear is what we genuinely think.”

He was also pressed on his ”f*** business” comments, but claimed that there no one in the Tory party ”can honestly be said to have done more to stick up for business, even in the toughest of times.”

Tory MPs will begin to whittle down the crowded field of candidates on Thursday, with the first round of voting expected tomorrow morning.