Mr Johnson, who will resign from No 10 on September 6, took a holiday earlier this month despite warnings of further inflation and the threat of a recession later this year.
The Times newspaper reports that Mr Johnson is spending a week-long holiday in the country, with locals spotting him shopping alongside wife Carrie in a supermarket in Nea Makri, a town to the east of Athens.
It’s all just one big party for Boris Johnson while the country struggles to pay their bills
On Monday, former Cabinet minister Brandon Lewis defended Mr Johnson, rejecting any suggestion that he had “thrown in the towel”.
He told LBC: “Even when you are not in the office in Downing Street you are working.”
He added: “He’s probably in about his second week [of] holiday in the last year or so, certainly this year. So while somebody is away, whether they are secretary of state or let alone the Prime Minister, they will be continuing to work.
“I can assure you he will still be going through inboxes, he will still be dealing with national security issues where relevant. Being out of the country does not mean the Prime Minister stops working.”
Labour hit out at the Prime Minister, accusing him of treating recent months as “one big party”.
A spokesperson said: “On the evidence of the last few months it seems to make little difference if the Prime Minister is in the office or on holiday as he has continually failed to meet the challenge of the Tory cost-of-living crisis. It’s all just one big party for Boris Johnson while the country struggles to pay their bills.”
Downing Street has been contacted for comment.