Boris Johnson has denied being involved in “dark arts” following claims his supporters were attempting to find a way to knock Michael Gove out of the race for the Tory leadership.
One of the remaining candidates will be eliminated when the result of the fourth ballot of Tory MPs is announced at around 1pm.
There has been widespread speculation at Westminster that allies of frontrunner Mr Johnson could lend support to Sajid Javid, who finished 13 votes behind Mr Gove in the last ballot, in order to force the Environment Secretary out of the race.
Mr Gove’s decision to stand for the leadership in 2016 torpedoed Mr Johnson’s own campaign then, and the wounds have not healed.
But asked if he knew anything about dark arts when he arrived to vote, Mr Johnson told reporters: “No.”
Jacob Rees-Mogg, a supporter of Mr Johnson, said any “dirty tricks” by supporters of the former foreign secretary to try and knock Mr Gove out of the contest would be “silly”.
He told the Press Association: “I think people should always vote for the candidate they support.
“It is really silly to try and game elections because you can find that your candidate then loses.”
Former Tory leader Iain Duncan Smith, who also backs Mr Johnson, said the only “dirty tricks” were that “MPs never tell you always the truth when it comes to leadership elections”.
Asked if there was an underhand campaign against him, Mr Gove said: “I know there is a campaign for me with wonderful, wonderful people.”
He added: “I’m looking forward to being in the final two as a result of the strong support I have across the party.”
Foreign Secretary Jeremy Hunt, who has finished second in all the votes so far, said he was “confident, but not over-confident”.
After a final fifth round of voting later on Thursday, just two candidates will remain in the race and they will go on to a ballot of 160,000 Tory members to choose the next prime minister.
Cabinet minister Amber Rudd, who supports Mr Hunt, urged Mr Johnson to publicly come out against “game-playing” in the Tory leadership race.
She old BBC Radio 4’s Today programme: “I find all this conversation about lending votes rather discrediting of the system.
“I would really call on Boris himself to repudiate the information that is coming out of ‘friends of Boris’, saying this, saying one thing.”
The Work and Pensions Secretary added: “This is a serious moment. We don’t need that sort of game-playing going on in Parliament.”
Eliminated Tory leadership candidate Rory Stewart has said he will not publicly back any of the remaining contenders on Thursday.
He has scheduled his own “thank you rally” for supporters at the time the announcement of the final pair is due to be made at 6pm.
He tweeted: “I will not be declaring for anyone today – but I will be voting.”
Former foreign secretary Mr Johnson appears certain to make it through to the next stage of the process, having topped the ballot in each of the three rounds of the contest so far and securing the votes of 143 of the 313 Tory MPs.
In the third round, Mr Hunt secured 54 votes, Mr Gove 51 votes and Mr Javid 38.
Amid speculation that Mr Javid has his eyes on becoming Mr Johnson’s chancellor, the Home Secretary insisted his sights were set on Number 10 and he was “in it to win it”.
Mr Hunt said he was the person best placed to take on Mr Johnson in the members’ ballot, promising to put his “heart and soul” into the contest.
“The stakes are too high to allow anyone to sail through untested,” he said.