The Conservative Party has to undergo “another period of modernisation if it’s going to survive”, the frontrunner in the race to be its candidate for London mayor has warned.

Samuel Kasumu, a former aide to Boris Johnson, said he was the only Tory in the contest who was able to reach out to a city as cosmopolitan as the capital.

In an interview with HuffPost UK, the 35-year-old also called on the party to have a “much more nuanced conversation about immigration”.

His comments will be viewed as a swipe at home secretary Suella Braverman, who has been criticised for saying the UK faces an “invasion” of immigrants.

Labour’s Sadiq Khan is hoping to win a third term as London’s mayor in May next year.

His Conservative rival will be chosen by party members from a shortlist of three on July 19.

Kasumu is up against technology minister Paul Scully, veteran London Assembly member Andrew Boff and David Cameron’s former deputy head of policy, Daniel Korski.

Kasumu – currently a councillor in Welwyn Hatfield – quit Downing Street in April 2021 amid a row over the government’s Commission on Race and Ethnic Disparities report, which was branded a “whitewash” by some anti-racism campaigners.

He said the party had to “appeal to folks you might not even be ideologically aligned with” if it wanted to win.

“The first thing you have to say is, London is the most cosmopolitan city in the world,” he said.

“How do you reach all these groups? I think I am best place to do that. There are probably only two or three people in the whole Conservative movement who are able to do that.

“I’m the only one who is going to make the shortlist who can do that job.”

He added: “The Conservative Party is going to have to undergo another period of modernisation if it’s going to survive.

“Even if you shut the borders today, there are an increasing number of people from minority ethnic backgrounds, and in London specifically, nearly half the population were born outside the country.”

Rishi Sunak has made stopping asylum seekers crossing the Channel one of his five pledges to voters amid concerns that the government has lost control of the immigration system.

Braverman has also come in for severe criticism over the language she has used when talking about the issue.

“I can’t tell the home secretary what to say or how to articulate that,” Kasumu said. “But I think we need to have a much more honest conversation about immigration… a much more nuanced conversation about the subject.

“Most people are quite sensible about immigration. They recognise that immigrants can and will continue to play a role in allowing us to function as a society.”

Kasumu also warned Sunak that time was running out for him to set out his vision of the kind of country he wants to build.

The prime minister set out his five pledges to voters in January – stopping the boats, halving inflation, cutting waiting lists, growing the economy and reducing the national debt.

But Kasumu said: “If I’m advising the prime minister – which I’m not anymore – I would say, I get it, five pledges, you need to demonstrate competence, you need to earn the right to be heard again.

“It’s a bit like what I’m saying in London, you’ve got he earn the right to be heard and then at some point, sooner rather than later, we need to have a very clear vision about what the next iteration of a Rishi Sunak premiership could look like.”

“They don’t have a lot of time,” he added. “We have got party conference after the summer. Maybe that’s the time to start to articulate what a Rishi Sunak vision looks like.”

Just as the national polls show Labour ahead, the Conservatives are fighting an even stepper uphill battle in London, with a YouGov poll in April suggesting Labour holds a 40-point lead in the capital.

But Kasumu said: “If we win in London then all bets are off.”