Boris Johnson owes the public “clear answers” over whether he has taken cocaine, a rival for the top job has said.

Mark Harper said the Tory leadership frontrunner must face questions on his past and that it was “essential” he stop hiding from public scrutiny. 

The ex-chief whip said one of the reasons he entered the race was to “make sure that the winner is properly tested”, and in comments aimed at Johnson said: “If you’ve got nothing to hide, you won’t mind answering questions.” 

The demand for transparency comes after candidates’ past use of drugs became a central issue in the race to succeed Theresa May as PM. 

Michael Gove, one of 11 MPs in the running, faced calls to drop out of the race after he admitted using the class A drug cocaine before he entered politics – something the minister admits he could have been jailed for. 

But questions hover over Johnson, the clear favourite for the job who has been avoiding media questions, and the extent of his own drug use. 

Johnson admitted to GQ magazine in 2007 he tried cocaine and cannabis as a teenager at Oxford.

“I tried it at university and I remember it vividly,” he said. 

In an appearance on the BBC Two programme Have I Got News For You he cast doubt over the issue, however. 

“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,” he said.

On cannabis, he also told the show: “There was a period before university when I had quite a few (cannabis joints). It was jolly nice.” 

Harper, who launched his bid for the leadership on Tuesday, was asked whether Johnson must now come clean. 

He said: “I think all candidates in this race owe the public clear answers. I’ve been asked about any drug use [experience] I’ve had and I’ve been very clear I haven’t taken any illegal drugs in my entire life and I come from a background frankly where I was brought up where people didn’t do that sort of thing and I don’t get invited to those sort of parties and I don’t hang out in those sorts of circles.” 

Harper added “there are consequences” for drugs offences which often fall on those “less privileged” in society.

And in a message to Tory MPs, who are weighing up which of the 10 potential leaders to support, he warned that the winner’s history could come back to bite the Conservative Party during a future general election campaign.

“The question colleagues in parliament need to ask themselves is: if you are not prepared to answer these questions now, you are going to get asked all of these questions in a general election campaign, and if you’re a Conservative MP in a marginal constituency, I’d want the person who is leading my party to be answering those questions now,” he said. 

“I don’t want them to be answering these questions in three years’ time when it is going to be my marginal constituency that is up for grabs. 

“So I think everyone has to answer those questions and my colleagues will make a judgement about how frank they have been. 

“If you’ve got nothing to hide, you won’t mind answering questions.” 

Johnson has yet to agree to take part in TV debates between candidates. 

Harper told journalists he would take part, adding: “I think it is important that we have that level of scrutiny.”

And in a further barb aimed at Johnson, he said “some of the candidates are saying they are fantastic campaigners” and that “they can reach parts of the country that others can’t reach”, adding: “Well: show us.” 

The Forest of Dean MP said: “The winner of this race is going to become the next prime minister of the United Kingdom and I think the public are entitled to see us in action, see us answering questions and frankly it is also an opportunity. 

“Effectively, this is the start of the 2022 general election campaign because at the end of this process the public is going to have taken a view about the person that becomes prime minister.” 

Harper also hit out at fellow candidates’ claims that the UK can leave the EU with a renegotiated deal on October 31, saying the plan was “not credible”. 

The outsider in the race also portrayed himself as a candidate who is untainted by the divisions of Theresa May’s government, having not served in office under the PM. 

He will say the country needs leadership from someone “who wasn’t in the room when mistakes were made”.

A return to “proper, functioning Cabinet government” can “only be enforced by someone who hasn’t been party to what has been called the ‘worst example of ill-discipline in Cabinet in British political history’”.