A counter-terrorism chief has said he was “surprised” Boris Johnson’s phone number was easily accessible online and suggested he may be warned to take security advice seriously.

Metropolitan Police Assistant Commissioner Neil Basu said on Friday there is “no shortage” of advice from security experts around the Prime Minister but said “you have to want to take it”.

He suggested Cabinet Secretary Simon Case might warn Mr Johnson to take the advice seriously, amid concerns he had left himself vulnerable to covert activity by hostile states.

It was revealed on Thursday that the Prime Minister’s number has been freely available on the internet for 15 years, having been published in a think tank press release and never deleted.

Lord Ricketts, the UK’s first national security adviser, said the disclosure could mean “thousands” of people have Mr Johnson’s mobile number, putting him at “increased risk”.

But Chancellor Rishi Sunak has insisted “all security protocols” were followed.

The presence of his mobile number online emerged amid a row over lobbying and concerns business leaders were able to personally message him to ask for interventions.

Reports have said Mr Case had advised Mr Johnson to change his number because of concerns it is widely known.

Asked about the presence of the Prime Minister’s number online, Mr Basu told Times Radio: “I think probably everyone is surprised by that. But I’m not sure I can say much more than that.

“The reality is, the Prime Minister is surrounded by security experts. You know, there is no shortage of advice about what he should and could do.

“You have to want to take that advice, but we work very closely with the Government security group, and they have access to all of the minds of the UK intelligence machine, including counter-terrorism policing, and we will provide that, but you have to want to take it.”

I think one would be worried if a hostile state who had sophisticated capabilities had the mobile phone number itself

Asked if he would tell Mr Johnson to take the advice seriously, Mr Basu said: “I wouldn’t, but the Cabinet Secretary might.”

Labour leader Sir Keir Starmer said the wide availability of the Prime Minister’s mobile number represents “a security risk”.

“He was warned about it – that tells its own story,” he told reporters in Hull.

“I changed my number when I became director of public prosecutions, and I’ve kept my number secure since then.”

Lord Ricketts, who advised former prime minister David Cameron on national security matters, said it is in Mr Johnson’s “own interest to be much more digitally secure than seems to be the case now”.

Asked if there could be security concerns, the peer told BBC Radio 4’s Today programme: “I know that modern systems like WhatsApp are end-to-end encrypted; nonetheless, I think one would be worried if a hostile state who had sophisticated capabilities had the mobile phone number itself.

“That must increase the risk that they’re able to eavesdrop on some at least of the communications that are going on, and possibly other non-state actors as well, like sophisticated criminal gangs.”

As well as the fresh disclosures in the row over the Conservative Party leader’s phone contacts, there were also new calls for further investigations into how renovations to Mr Johnson’s flat in 11 Downing Street were funded.

The Prime Minister looked to shift attention away from the controversy as he insisted he was “laser-focused” on delivering the country’s priorities as the parliamentary session came to an end on Thursday.

But senior Labour MP Dame Margaret Hodge, who chaired the Public Accounts Committee spending watchdog, asked the Parliamentary Commissioner for Standards to investigate his conduct over the flat revamp.

In Dame Margaret’s letter to commissioner Kathryn Stone, she said it had been suggested that as much as £200,000 was spent on the redecoration, with some funds originating from the Conservative Party and Tory donors.

Inquiries into how the redecoration work was funded are being carried out by head of the Civil Service Mr Case, independent adviser on ministers’ interests Lord Geidt and the Electoral Commission.

The Prime Minister has insisted the row over refurbishments to the flat where he lives with fiancee Carrie Symonds is a “farrago of nonsense”, adding: “I don’t think there’s anything to see here.”

He says he “personally” paid for the renovations but has refused to say whether he received an initial donation from the Conservative Party to cover the costs.

The coverage about the flat does not, according to YouGov, appear to have damaged the Prime Minister in the eyes of the voters, with the Tories extending their lead over Labour from 10 to 11 points this week despite polling showing that only 14% of the public had not heard about the issue.

Research manager Patrick English wrote in a blog post: “If the crisis deepens for Johnson and he finds himself facing potential charges and prosecution, this may well tip the balance.

“For now, however, the stories are certainly sticking in the minds of the public, but don’t appear to be changing them.”