It is a glittering annual dinner in honour of the Conservative party’s most successful leader and, on the 100th anniversary of Margaret Thatcher’s birth, one that is bigger than ever.

Yet as Tory grandees, celebrity backers and wealthy donors prepare to sit down at a gala dinner at London’s Guildhall on Monday evening, a battle for her legacy is under way between the party she once led and the insurgent threat to its survival, Reform UK.

Boris Johnson will make a keynote speech at the event, the Guardian has been told by its backers, joined by the main speaker, Thatcher’s son, Mark, and guests including Joan Collins, Julian Fellowes and Jeffrey Archer.

But a notable absentee will be Kemi Badenoch, as the current Tory leader struggles to stave off the threat of extinction looming over the party Thatcher led to three consecutive election wins.

“There is a competition as to who is best to carry forward her legacy,” said the event’s main sponsor, Mohamed Amersi, who has donated £500,000 to the Tories in the past but, like other donors, has been backing Nigel Farage’s party more recently.

He said: “There’s no doubt that she would have been sympathetic to much of what Reform are saying, though at the end of the day she was still a reluctant remainer.”

Amersi said it would “take time but we will find somebody”, adding there was “not a chance in hell” that Badenoch was the answer to the Tories’ challenges.

Badenoch is expected to appear instead at an official party fundraiser dinner in Greenwich. She has likened herself to Thatcher, telling Tories at their annual conference that she too was being written off like Thatcher once was.

While Thatcher’s face was a constant presence at this year’s Tory conference, party sources say even many on its rightwing pay lip service to her memory while regarding her small-state philosophy as out of kilter with Britain in 2025.

“She is very important though when it comes to mobilising donors,” they added.

Many of those donors and those who were close to Thatcher are preferring to go to more private dinners. Among those is Sir Rocco Forte, the Brexit-backing hotelier who helped to fund Johnson’s election win and knew Thatcher. He remains tentatively supportive of Badenoch.

He said: “I don’t think there really is a standard bearer for [Thatcher] at the moment in politics. I would say the Conservative party itself now is divided between a big faction which I would not even call Conservative and Kemi Badenoch, who I do still think is a serious believer in what Margaret Thatcher advocated.”

Tickets for the sold-out Guildhall event ranged in price from £195 for an individual up to £3,000 for a VIP table. While there will also be Tory grandees such as Michael Howard at the dinner this year, they will be joined by Reform UK figures including its deputy leader, Richard Tice.

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Speaking to the Guardian at a time when Reform is treading a line poaching both Tory and Labour voters, he picked his words carefully when asked if Thatcher would be a Reform supporter today, but still likened her to his party leader.

Tice said: “She was always about challenging the status quo, with strong conviction, and in a sense that is Nigel’s philosophy. The key thing is that she was a transformational leader of a country that was in trouble then, as it is now.”

Events elsewhere include a dinner at the Carlton Club and an 80s-themed disco that Conservatives are holding on Monday at a location described as an ‘‘iconic central London nightclub”.

Organisations that have been putting on events include the Centre for Policy Studies (CPS), the thinktank Thatcher co-founded and which provided the intellectual stimulus for her policies.

Its director, Robert Colvile, said the Tory conference had probably gone “too far” by displaying her dresses in glass boxes. But he insisted that her political echoes remained powerful, pointing to a CPS event that was packed out by delegates.

“If you look at the polls, she is consistently viewed as the most effective leader we have had. Her self-appointed mission was to turn around a Britain that was in decline and we are pretty obviously in that situation again now. Even people like Keir Starmer, while disagreeing with her policy, admit that they admire her for bringing change.”