Boris Johnson’s endorsement of Conservative election candidates “will make a difference”, Prime Minister Rishi Sunak has said.

Mr Johnson is reportedly putting his name to thousands of letters to to be sent out to voters urging them not to support Nigel Farage’s Reform UK on 4 July.

The former PM has also recorded videos endorsing some Tory candidates.

But he has yet to appear on the campaign trail in person and is currently on holiday in Sardinia, according to his wife Carrie’s Instagram account.

Some Conservatives had hoped Mr Johnson would make headline-grabbing appearances at election events to persuade those who voted him into power in 2019 to stick with the Tories.

But relations with Mr Sunak remain strained, after Mr Sunak’s resignation as chancellor hastened Mr Johnson’s departure from Downing Street.

Mr Johnson does appear to be working with Conservative Party HQ to help the party shore up support, as it continues to trail Labour by a big margin in the polls.

In a letter to voters, Mr Johnson is expected to urge people not to vote Reform and warn that Labour could end up in power for a generation.

This is in line with the message being pushed by other senior Tories, as Mr Farage’s party gains ground in the polls.

Speaking on a campaign visit in North Devon, Mr Sunak was asked whether he felt “let down” that Mr Johnson has not been active on the campaign trail.

He insisted his predecessor is supporting his efforts to win the election.

“It’s great that Boris is supporting the Conservative Party, I very much welcome that. He’s endorsing many candidates in videos and letters which have been coordinated by the campaign and I know that will make a difference.

“And of course every week he’s making the case in his column… and I’m glad he’s doing that.”

Mr Johnson has been signed up by GB News to present coverage of the general election and the US elections, although he has yet to make an appearance on the channel amid claims he is concentrating on finishing two books he is under contract to write.

The former PM has been using his weekly Daily Mail column to make sure “everyone understands what the Labour government would do to this country and why it’s important that everyone votes Conservative”.

On 14 June, Mr Johnson claimed a large Labour majority would be “a nightmare” which only voting Tory could prevent.