Security experts today warned Boris Johnson was at "increased risk" of being targeted by hackers and enemies after his private mobile phone number was online for 15 years.
The Prime Minister's personal contact details were featured at the bottom of a press release issued by a think-tank in 2006 when he was Shadow Higher Education Minister.
His mobile number was given in footnotes as someone who could be contacted because he helped write a study.
The document was deleted from the web overnight on Thursday after gossip email Popbitch discovered the details.
Former UK national security adviser Lord Ricketts said there was an "increased risk" of potential action by hostile states or criminal gangs if a phone number was widely available.
Asked if there could be security concerns, he told the BBC that if the number has been "widely available", it cannot be ruled out that hostile states or criminal gangs could have accessed it.
"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," he said.
"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."
The peer said it was in Mr Johnson's "own interest to be much more digitally secure than seems to be the case now".
He added: "I'm talking really of the most senior politicians in sensitive positions, whose phone conversations might well include sensitive material, commercially sensitive material, people trying to lobby them for favours, or tax advantages, or talks with foreign leaders.
"And there, I think you do have to accept, just as you do - you can't just walk around on your own and talk to anyone you like - equally you shouldn't be in a position where anyone who once had your phone number can get to you when you are a prime minister.
"And that's one of the inconveniences of being Prime Minister but it's for their own sake and their own protection really, that access to them ought to be controlled and monitored.
"So that there can be no suspicion of favours asked for and done or the kind of things that we are now seeing with the exchanges that we see with James Dyson.
"I think it's for the Prime Minister's own interest to be much more digitally secure than seems to be the case now."
Senior Labour MP Kevan Jones, who sits on Parliament's Intelligence and Security Committee and is a former Defence Minister, said: “This is extremely worrying with massive security implications. It needs to be investigated as a matter of urgency why this has not been addressed before now. It leaves the prime minister very vulnerable.”
However, Home Office Minister Victoria Atkins claimed the PM “knows his responsibilities” over national security – and fired a broadside at the media for running the story.
"The Prime Minister, more than anyone, knows his responsibilities when it comes to national security," she told Times Radio.
"I'm slightly surprised that a national broadcaster felt it appropriate to advertise the fact that that mobile phone is on the internet, if indeed it is."
