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 leader Sir Keir Starmer said it was "a serious situation [that] carries a security risk".
But Chancellor Rishi Sunak said that, as far as he was aware, "all security protocols have been followed".
It appears the number has been switched off but Downing Street still has not confirmed if the number will now be changed.
Questions have been raised in recent weeks over who has access to ministers' personal phone numbers, amid a wider row about lobbying and influence.
The PM had reportedly been advised by senior officials to change his number, but this has been denied by Downing Street last week.
Sir Keir said: "A lot of people will be concerned, not just about who has got the number but who has been using it.
"What has come out in recent weeks is [about] privileged access - those who can WhatsApp the prime minister for favours - and this all is further evidence that there is one rule for them and another rule for everybody else."
The Labour leader also said he had changed his number when he became director of public prosecutions in 2008, adding: "I have kept it secure since then."
But Mr Sunak said what made the PM "special" was that he was "incredibly approachable".
He added: "You see it whenever 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."
Mr Sunak also revealed he had not changed his own personal phone number since becoming chancellor last February.
The think tank press release with the PM's number on it 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".
But earlier this week, the UK's top civil servant, Simon Case, told MPs that officials "put security packages around all the communications that ministers have by whatever means".
He added: "The level of encryption and standards of security on everybody's mobile phone increasing all the time."
Labour MP Rachel Hopkins said the incident had implications for security, lobbying and the risk of blackmail.
But Home Office Minister Victoria Atkins said the PM was "aware of his responsibilities" on national security.
She told the BBC that she had "complete and utter confidence" that Mr Johnson and his advisors knew what needed to be done for him to fulfil those "duties".
It was revealed earlier this month that the prime minister had contacted businessman Sir James Dyson at the start of the pandemic.
In an exchange of text 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.
In responding to the urgent call for help at the start of the pandemic, Sir James also wanted to be clear about complying with rules and protecting the business from any extra costs or liabilities.
But Labour leader Sir Keir Starmer said it was wrong that people "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 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.