The Prime Minister could have put himself at an “increased risk” of snooping and criminal activity, according to a former national security adviser, after it emerged his mobile number had been openly available online for 15 years.

Lord Ricketts said the revelation that, until this year, Boris Johnson’s contact number was still listed on the bottom of a 2006 press release on the internet meant “thousands” of people could have his details, possibly placing him at risk of hostile state action.

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.

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

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.

Lord Ricketts, who served as the first UK national security adviser under David Cameron, said it is in Mr Johnson’s “own interest to be much more digitally secure than seems to be the case now” following the reports that a press release dating back to when he was shadow higher education minister could be accessed on the internet in 2021.

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.

“So, there is no way of knowing whether that’s true, but there must at least be an increased risk if the number is widely available.”

Attempts to call the number on Thursday night were met with an automated message saying the phone was “switched off” and an invitation to “please try later or send a text”.

The Conservative Party leader’s use of his mobile phone has been in the spotlight after text message exchanges with entrepreneur Sir James Dyson and Saudi Crown Prince Mohammed Bin Salman were leaked.

Home Office minister Victoria Atkins insisted that, despite the fresh embarrassment for Mr Johnson over his mobile number being in the public domain, the No 10 incumbent “more than anyone, knows his responsibilities when it comes to national security”.

Ms Atkins added that she believes the public are not “particularly interested” in the issue.

It follows calls for further investigation into how the lavish 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 people’s priorities as the parliamentary session came to an end on Thursday.

But Labour sought to pile more pressure on, as senior Opposition 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.

“It is therefore unacceptable that the Prime Minister refuses to publicly declare exactly where the upfront costs for this redecoration came from, exactly how much these costs amounted to, and who the Prime Minister is repaying or has repaid to cover these costs,” said Dame Margaret.

A Commons rule change means Ms Stone is once again allowed to publish the names of MPs she is probing for breaches of the code of conduct.

But the commissioner said she will not reveal the names of any MPs under investigation until after May’s elections.

Inquiries into how the redecoration work was funded are already 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 lavish refurbishments to the flat where he lives with fiancee Carrie Symonds is a “farrago of nonsense” and “I don’t think there’s anything to see here”.

He has said 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.

Lord Lilley, a former deputy Tory leader, said people are getting “worked up over nothing” over the upgrade.

He told Times Radio: “The Labour Party is naturally trying to exploit this, but my point is nothing bad has happened, no public money has been siphoned off to private benefit.”

Any loan, he added, was “clearly temporary because the Prime Minister has ended up paying out of his pocket”.