Boris Johnson defied orders from Tory chairman Brandon Lewis not to use the Peterborough by-election as a vehicle for his leadership campaign, HuffPost UK has learned.

The former foreign secretary and other contenders to replace Theresa May were told that to focus on anything other than helping local candidate Paul Bristow would be unhelpful.

A leaked email from the party chairman also suggested that any attempt to use Bristow in any leadership material could add to election expenses for the by-election.

However, Johnson did indeed feature the candidate in his campaign launch video, which was put out just days before polling day.

A string of other possible leadership contenders visited the seat, including Michael Gove, Matt Hancock, Esther McVey and Sajid Javid, but none of the others used the occasion to boost their own campaigns.

In the end, Bristow came a poor third in the by-election, behind both Labour’s Lisa Forbes and the Brexit Party’s Mike Greene.

Johnson’s video shows him asking voters on the doorstep if they would switch to back the Conservatives if the UK got out of the EU with or without a Brexit deal with Brussels. “Yes, yes,” one pensioner tells him.

By-elections have strict rules on spending, with a limit of up to £100,000 for each party’s candidate designed to create a level playing field.

Lewis, who last year incurred the wrath of Johnson supporters when he launched an inquiry into his remarks about Muslim women resembling ‘letterboxes’, sent an email to all potential leadership teams on May 31.

The message, which has been passed to HuffPost, states ‘focus on anything other than our local message (ie the leadership campaign) will not help us in this by-election’.

It adds: “If a leadership campaign decides to bring other resource (eg video camera crew) they are NOT to be used for purposes of the by-election campaign. No footage should feature the by-election candidate either”.

Despite the warnings, Johnson included Bristow in his glossy campaign video, in a scene where he met Peterborough voters.

“He was warned not to campaign, but once again couldn’t resist putting himself first. He thinks the rules don’t apply to him,” one party insider said.

A source close to Johnson said that he had not broken any party rules and had passed his paperwork and costs for his campaign visit to Conservative Campaign HQ (CCHQ).

He had not ‘promoted’ the local candidate in his own leadership material and was unlikely therefore to have incurred extra expenses, they added.

In one speech captured on video, Johnson appeared alongside Bristow to declare: “If we want a governing party that can deliver on the promises that we made to the British people then you’ve got to vote for Paul Bristow and the Conservative Party, because we are the party that is going to take Britain out of the European Union and deliver on that pledge.”

He then went on to attack “Jeremy Corbyn and his cadre of krypto-Communist, Hugo Chavez admiring, anti-Semitism condoning, apologists for the Kremlin”, declaring they should “never get anywhere near the keys of Downing Street”.

Wayne Fitzgerald, chairman of Peterborough Conservative Association, defended Johnson’s trip.

“Boris took some time to campaign but he was very generous to the candidate,” he told HuffPost UK.

“He went knocking doors, extolled the virtues of the candidate. In between all that he may well have captured some footage that he could use.

“When Sajid Javid turned up you would have thought Bono had arrived. When Boris turned up we took him to a local shopping centre and you would have thought the Pope had arrived.

“Inevitably he steals some of the limelight but it would be a bit odd if he didn’t. Everybody knows who Boris Johnson is but not everybody knows who Paul Bristow is. By association, Paul got the benefit of some, shall we say, Boris ‘magic’.”

Johnson, who remains the front-runner in the Tory leadership race with more MPs publicly backing him than anyone else, is due to make his launch speech at an event on Wednesday.

A spokeswoman for Conservative Campaigns HQ was approached for comment.