The Chancellor has said “all security protocols” were followed after a security expert warned revelations that Boris Johnson’s phone number was easily accessible online could put him at increased risk of snooping.

Rishi Sunak played down fears that the Prime Minister had opened himself up to possible covert activity by hostile states and exploitation by criminal gangs after it emerged his personal contact details had been accessible on the internet for the past 15 years.

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”.

I think a lot of people will be concerned, not just about who's got the number but who's been using it

But senior Cabinet minister Mr Sunak, speaking on a by-election campaign visit to Hartlepool, said: “As far as I’m aware, all security protocols have been followed.

“Part of what makes the Prime Minister special is that he is an incredibly approachable individual.

“You see it wherever he is out and about – people feel they can relate to him, they can talk to him, they can tell him what’s on their mind.”

It emerged on Thursday that Mr Johnson’s contact number was still listed this year at the bottom of a 2006 press release dating back to when he was shadow higher education minister.

It follows a row in recent weeks over so-called Government-by-text after it emerged that lobbyists and others from the business world had personally messaged the Prime Minister in a bid to get him to intervene in issues.

Downing Street did not deny reports earlier this month that Cabinet Secretary Simon Case advised Mr Johnson to change his long-held phone number because of concerns over how many people had his direct contact details.

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 think a lot of people will be concerned, not just about who’s got the number but who’s been using it.

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

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

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.”

Former head of MI6 Sir Richard Dearlove said the security concerns over Mr Johnson’s phone were “not a unique problem” due to the “nature of political life”.

In an interview with LBC radio, reportedly recorded before the Prime Minister’s number was spotted online, Sir Richard said: “Politicians and phones have always been a problem.

“It is not a unique problem to any single politician. Every single politician I have ever known, foreign and British, have used their phones in a way which probably is unwise.

“That’s the nature of political life. It’s not to do with the behaviour of politicians.”

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 asked the Parliamentary Commissioner for Standards to investigate his conduct over the flat revamp.

In Dame Margaret’s letter to commissioner Kathryn Stone, the former chairwoman of spending watchdog the Public Accounts Committee 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.

Sir Keir, pressed on what his party wanted the commissioner to ask Mr Johnson, said: “It’s not a difficult question – ‘who initially paid for the refurbishment of your flat?’.”

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.”