British Prime Minister Boris Johnson's personal mobile phone number has been on the internet for 15 years, according to media reports, raising questions about national security.

The number, which the BBC reported was still used by the Conservative Party leader, appeared on a press release from 2006 when Johnson was an opposition MP.

The revelation comes as Johnson is under scrutiny over a lavish makeover of his Downing Street, and close ties between big business and UK politicians, including text messages he exchanged with industrialist James Dyson.

Former national security adviser Peter Ricketts told the BBC that hostile states and criminal gangs could have access to Mr Johnson's contact details, adding that it was an "elementary security precaution these days" for a leader to change their number.

He said it would be in Mr Johnson's "own interest to be much more digitally secure than seems to be the case now... 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".

Home Office minister Victoria Atkins insisted that Mr Johnson was aware of his responsibilities on national security.

"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."

She added that she believed the public were not "particularly interested" in the issue.

When the number was called by AFP today, there was an automated message indicating that the phone was disconnected.

Mr Johnson is under pressure to answer who initially footed the bill for a reported £200,000 (€230,000) refurbishment of his Downing Street flat.

The Electoral Commission, which regulates the accounts of political parties, announced on Wednesday that it was opening an investigation into the issue.

It is not the first time a political leader has had his personal number in the public domain.

In 2014, then Taoiseach Enda Kenny's mobile number was published on social media sites and despite the posts containing the details being deleted in accordance with privacy policies, the mobile number was shared.

Additional reporting PA