Boris Johnson’s endorsement of Conservative candidates has been co-ordinated by the party’s General Election campaign and “will make a difference”, the Prime Minister has said.

Rishi Sunak’s predecessor-but-one is reported to have put his name to hundreds of thousands of letters due to be sent out to voters, urging them not to support Reform UK and warning that Labour could end up in power for a generation.

He has also recorded videos endorsing some candidates, but has so far not appeared on the campaign trail in person and is currently on holiday in Sardinia, according to a post on his wife Carrie’s Instagram account.

Mr Sunak told broadcasters on Tuesday that he welcomes the contribution of the former premier, who quit Parliament last year after being found to have misled the Commons over Partygate.

He said: “He is endorsing many candidates in videos and letters which have been co-ordinated by the campaign. I know that will make a difference.”

The extent of Mr Johnson’s involvement in the Tory campaign was subject to much speculation prior to the election, with some figures urging Mr Sunak to make more use of his predecessor while others suggested strained relations between the pair would make that complicated.

The admission that Mr Johnson’s interventions have been co-ordinated with the Conservative Party is the first indication that the former premier is involved with the official campaign rather than providing ad hoc endorsements of individual candidates.

Mr Sunak added that Mr Johnson is also using his column in the Daily Mail to make sure “everyone understands what the Labour government would do to this country and why it’s important that everyone votes Conservative”.

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

The argument echoes those made by other Conservative figures in an effort to counter the threat from Reform UK, with politicians from the Prime Minister down saying a vote for Nigel Farage’s party would only hand Sir Keir Starmer’s Labour a larger majority.