Tony Blair has been accused of violating COVID-19 restrictions, reported The Sunday Telegraph.
Those failing to follow the rules could be liable for a £1,000 fine, or potential prosecution.
Mr Blair had asked Whitehall officials for exemption from the rules, but was not issued with a formal letter which would have allowed him to avoid the two-week quarantine, according to the newspaper.
Blair, 67, is cited as claiming he was advised to follow rules on attending "international conferences" when he flew to the USA to attend a ceremony at the White House as Israel signed agreements establishing formal relations with Bahrain and the UAE.
However Blair, who stepped down from his No 10 post role as Middle East envoy in 2015, is considered a private citizen.
Government guidance states that diplomats and representatives at worldwide conferences are granted "privileges and immunities", meaning they are not required to self-isolate if their work is confirmed as essential.
The ceremony came after US President Donald Trump and his administration brokered the historic deal, which was strongly opposed by Palestinian leaders. He was pictured with other guests on the South Lawn of the White House, holding a face mask in his hand as he posed for pictures with U.S. political figures including Frank Luntz, the veteran pollster, and Kevin McCarthy, the Republican leader in the House of Representatives.
The ongoing fight against the coronavirus and stories on Tony Blair and MPs possibly flouting pandemic guidelines feature prominently in the nation's papers.
Mr Blair flew to Washington DC on September 14 and returned on September 16, according to the Sunday Telegraph, which reports he was seen leaving Harry's Bar in Mayfair on September 26.
"He (Mr Blair) was not refused exemption by the UK Government", the spokeswoman said.
The spokesman also insisted Blair "posed no risk to anyone" as he was tested before his departure, on arrival at the White House, and again several times since returning to the UK.
A Foreign, Commonwealth and Development Office (FCDO) spokeswoman said: "The FCDO provides travel exemptions to diplomats travelling on business relating to the interests of the United Kingdom, representatives of worldwide organisations, and their families and dependents".