A firm that specialises in digging dirt on political candidates has been circulating a “dossier” on James Cleverly that appears to have been commissioned by one of his Tory rivals or a third party trying to influence the contest. Despite the Conservative party’s efforts to ensure a clean competition with candidates banned from briefing against or criticising each other too harshly, Cleverly is believed to have been targeted at arm’s length by an opponent. Under a “yellow card” system, the Tories have said they will give public warnings to any candidate attacking or backbiting against their rivals. In the dossier, the research claimed that Cleverly had exaggerated elements of his former business career and downplayed the extent of his family’s links to a south-east London private school Colfe’s. However, it did not provide any damning evidence against him, with some of the sources given simply pointing to Cleverly’s roles in school plays and a parody write-up of a 1987 geography field trip on which: “Everybody was jolly chuffed because all of our rooms were within sneezing distance of everybody else – except for J Cleverly who was banished to the tower because his boots were wet.” The firm declined to disclose its client, citing the confidentiality of its agreement. Cleverly was joint third in the race behind Robert Jenrick and Kemi Badenoch after a vote of MPs this week, while Tom Tugendhat shared third position. A source close to the Cleverly campaign said: “You only try to do a hit job on people you’re scared of. We wish them well.” His spokesperson added: “James is running a clean campaign and will continue to do so.” Spokespeople for Jenrick, Badenoch and Tugendhat denied that they had anything to do with the dossier that had been circulated. So-called opposition research – digging on political candidates by their rivals – is frequent in the US but less so in the UK. One person involved in the “opposition research” industry said it was not uncommon for this to happen behind the scenes in Tory leadership contests but that clients typically had very varied goals in what their were trying to achieve – and were not always the rival candidates themselves. They said the information was sometimes just used by campaigns as they were talking to MPs who they wanted to attract, or at other times might be commissioned by corporate or political third parties who have a particular problem with a contender becoming a party leader or getting a specific job. The strictures put on the campaign by Conservative party headquarters, banning direct “blue-on-blue” attacks, may also have encouraged those supporting candidates to commission paid-for research aimed at discrediting others. Under the rules, Bob Blackman, who as chair of the Conservative 1922 Committee will oversee the race, has said he will give a yellow card and a public dressing down to any candidate who attacks a rival. “Constant backbiting and attacking colleagues” by Tory MPs during the last parliament was a key reason why the party did so badly in the general election, Blackman said at the start of the contest. Blackman said that if candidates “indulged” in personal attacks he would “get involved obviously to warn them and if necessary, issue a public statement”. “If a yellow card is issued and a statement made to the public and to the members, that a candidate has infringed on the rules, that will be extremely detrimental to their chances of getting elected,” he said. The rules also apply to Conservative MPs and other party members. The higher spending limits on the campaign could also have contributed to the phenomenon, with the candidates allowed to spend up to £400,000 on trying to become the next leader of the opposition. Earlier this week, Mel Stride, the former work and pensions secretary, became the second candidate to be knocked out of the race, after Priti Patel, a former home secretary, was previously eliminated. The four surviving candidates will now be subject to a “beauty contest” at the Conservative conference in early October, where they will make their case to party members. Jenrick, a former immigration minister, finished top of the closely contested ballot, collecting 33 votes, up five on the first round. Badenoch came second with 28 votes, up six on the previous round, while Cleverly and Tugendhat were joint third with 21 votes each. Cleverly’s number of votes stayed steady while Tugendhat’s tally went up by four. There were 119 votes cast by Conservative MPs in total, with no spoilt ballots. After their party conference in Birmingham, Conservative MPs will hold further rounds of voting to whittle down the list of candidates to a final two, with the winner to be decided by a vote of the grassroots membership. Members of Tugendhat and Badenoch’s campaigns claimed that their candidates had the most momentum going into conference. The fact that Cleverly’s number of votes did not change suggests he lost supporters, as two of Patel’s backers – Greg Smith and Alec Shelbrooke – publicly switched to him.