Boris Johnson's personal mobile phone number has been freely available on the internet for the past 15 years, it has been revealed.
It was published in a think tank press release in 2006, but never deleted - and appears to be the one the PM uses.Labour MP Rachel Hopkins said the availability of the PM's number had implications for security, lobbying and the risk of blackmail.
Downing Street has declined to comment on the story.
The BBC has tried to call the number several times since the story emerged, but the phone now appears to be switched off - only giving the option to send a text rather than leave a message.
Questions have been raised in recent weeks over who has access to ministers' personal phone numbers, amid a wider row about lobbying and influence.
Last week, No 10 denied the PM had been advised by senior officials to change his number.
The number's availability was first reported by the celebrity gossip email newsletter, Popbitch.The think tank press release was related to Mr Johnson's then-job as a shadow higher education minister - and MP for Henley - and invited journalists to contact him for further comment.
Two years later, he successfully stood to become mayor of London.
Former national security adviser Lord Ricketts warned hostile states with "sophisticated cyber capabilities" or criminal gangs could now have access to Mr Johnson's digits.
The crossbench peer also told BBC Radio 4's Today programme that he would have thought changing your mobile number when becoming PM would be an "elementary security precaution these days".
It was revealed earlier this month that the prime minister had a text exchange with businessman Sir James Dyson at the start of the pandemic.
In the messages, Mr Johnson promised to "fix" a tax issue for Sir James, whose company planned to work on building ventilators during the pandemic.
The PM said he had done the "right thing" in "shifting heaven and earth and doing everything I possibly could" to help increase supply.
But Labour leader Sir Keir Starmer said it was wrong that friends "who've got the prime minister's number can access him and ask about tax breaks".
The Daily Telegraph recently reported the UK's most senior civil servant, Simon Case, had suggested Mr Johnson change his number because it was too widely known.
And the Times reported last week that a Universal Credit claimant once contacted the PM directly "asking for help with their benefit claim" after they were passed the his number by a friend.
Ministers are issued with official mobile phones when they get a job in government, but they are allowed to keep their own personal devices.