London mayor Ken Livingstone has dismissed claims that some Labour supporters are trying to block Boris Johnson's bid to stand against him.
Boris Johnson is favourite to land the Tory nomination
The Tories are holding open primaries allowing all Londoners to help choose the party's candidate.
Two websites are urging Labour voters to take part, leading to claims they might choose a weaker candidate.
But a spokesman for the mayor said the story was "an invention" and no-one in Labour had any interest in the ballot.
The Conservatives insist safeguards are in place to prevent the primaries being scuppered by their political opponents.
Mr Johnson is one of four candidates hoping to win the Tory nomination. The other three contenders - Victoria Borwick, Warwick Lightfoot and Andrew Boff - are not well-known by the public.
In contrast, Mr Johnson, who quit his post as the Tories' higher education spokesman to enter the race, has received the lion's share of press coverage and is hot favourite to win the Tory nomination at the bookmakers.
Mr Livingstone has said the Henley MP would be a "formidable" opponent.
Any London voter can take part in the Tory primaries, which are intended to boost interest in the contest, but they have to call a £1.50 premium rate phone line in order to register.
The New Statesman is also encouraging its readers to vote in the Tory primaries on its website.
The site's editor, Ben Davies, said: "They have opened it up to everyone in London - even people who have no desire for the Conservatives to win, even those who have no interest in the political contest that will happen in 2008.
"So my feeling is, let's take advantage of this, let's vote.
"I like voting, I like elections. OK you have to pay £1.50 for this one, but nevertheless, regardless of your own political leanings, you can help the Conservatives by voting for one of their candidates.
"And if you want the Conservatives to lose, it's true you could vote for the one you considered the most hopeless."
Another website, Londontorynightmayor, has been set up with the specific intention of scuppering the Tory selection process.
It says: "It's a deeply sensible idea, and we'd encourage as many non-Tories as possible to get involved, just for a laugh.
"This website will soon list and profile the front-runners, and invite comments, with a view to picking the person who will make for the most entertaining contest."
The Conservatives believe the £1.50 charge will deter most mischief- makers, but shadow London minister Bob Neill said other safeguards were also in place.
He told the BBC News website: "We are having this run by the Electoral Reform Society.
"They have put in place a number of anti-fraud measures - and we are satisfied with them - that would enable us to detect any significant attempt to try and influence or skew the ballot.
"We reserve the right to disqualify fraudulent voters, as anyone would.
"If it became apparent that there was an attempt by some group to try and twist or distort the primary - to try and hijack it - that would be the most obvious demonstration that they are scared."
He said the idea of the primary was to "give all the candidates a chance to set out their stall and what I want to do is to make sure whoever is decided is the best possible candidate to beat Ken Livingstone next May."
Victoria Borwick, one of the four shortlisted candidates, said: "I can't believe that with seven million Londoners there is going to be much opportunity for a few Labour enthusiasts to scupper things."
A spokesman for Mr Livingstone said: "This story is an invention by the Tories, who are desperate to distract attention from the negative coverage that Boris Johnson's hard-line right-wing views are now receiving.
He pointed out that the "Torynightmayor" site had been set up months ago, even before Mr Johnson announced he was running for the mayorship.
And he said that the Conservative Party "should and will be left alone" to put forward Mr Johnson's views.
"They know that having a candidate with such known right-wing views will deeply damage the Tory party nationally as well as in London. Therefore no-one supporting Labour has any interest in interfering in their ballot."
Mr Johnson is on holiday in the US. His office said he was not contactable by telephone.
The four contenders will take part in hustings throughout September, with the winner to be announced at the Conservative Party conference on 26 September.