Rishi Sunak will meet leading Conservative Party donors on Monday evening as he seeks to mobilise financial support for the surprisingly timed general election campaign he triggered last week.

Sky News understands that the prime minister will have dinner with a small group of long-standing and more recent donors as the Tories target millions of pounds in fresh contributions to fund their push to retain power.

This weekend, Lord Spencer, the City billionaire, said he was giving £250,000 to the Conservative campaign, although a leaked party memo reported by The Times on Monday suggested it was so far struggling to raise money.

The dinner will take place as Labour prepares to unveil a list of senior business figures who are endorsing the party ahead of the 4 July election.

A draft letter circulated to private sector bosses late last week, and reported by Sky News, accused the Tories of presiding over an economy “beset by instability, stagnation, and a lack of long-term focus”.

Labour refused to comment on the identities of those who had signed the letter prior to its publication, although there was speculation that Sir Jim Ratcliffe, the Ineos founder and Manchester United Football Club joint-owner, had been invited to do so.

One Labour official denied that Sir Jim, a Monaco resident who this month said he thought that Sir Keir Starmer would do “a very good job at running the country”, was among the list of signatories.

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The draft letter, designed to aid Sir Keir’s bid to reposition Labour as the natural party of business, said the UK “has the potential to be one of the strongest economies in the world” but added: “”A lack of political stability and the absence of consistent economic strategy has held it back.”

“Labour has shown it has changed and wants to work with business to achieve the UK’s full economic potential,” it added.

“We should now give it the chance to change the country.”

A number of FTSE-100 chiefs, some of whom have traditionally signed pro-Conservative letters in the run-up to elections, are understood to have been approached to sign it.

One said it was “too political” for him to sign, but Labour allies insisted on Monday that the party had assembled an “impressive” list of signatories.

Sky News revealed last week that the Tories have contacted business leaders since Mr Sunak called the election, asking them to take part in broadcast media opportunities, provide quotes in support of manifesto pledges and host events and visits for cabinet ministers.

It was unclear on Friday whether the Tories would seek business signatures for a public letter similar to the one being prepared by Labour.

A Conservative Party spokesman declined to comment on Monday’s dinner with the PM or to identify those he was meeting.