The prime minister has refused to "get into details" about his father's trip to Greece during the coronavirus pandemic.
Foreign Office advice currently urges Britons against all but essential international travel.
Asked about the trip, the PM told LBC Radio: "I think you really ought to raise that with him. I am not going to get into details of family conversations.
"I think the overwhelming majority of the British people have understood what needs to be done and have been very prudent, and that is the right thing to do."
Transport Secretary Grant Shapps also defended the PM's dad.
Asked if Stanley Johnson was within his rights to fly to Greece, Mr Shapps said: "Yes. What isn't advice and is legally-restrictive, is the quarantine. That would apply to him as much as it would apply to anyone else."
Mr Johnson shared a video on Instagram of his plane landing in the Greek capital on Wednesday.
He also posted a picture of himself wearing a face mask, which appeared to have been taken in an airport.
The posts have since been removed from the social media platform.
Mr Johnson told the Daily Mail he was visiting the country on "essential business trying to COVID-proof my property in view of the upcoming letting season".
Asked about the trip, Mr Shapps told Sky News: "The advice says you have to make your own decision about whether you want to travel.
"What he won't be able to ignore, and this applies to everybody, anyone returning to this country from a country not on the list has to quarantine for 14 days.
"That would apply to him the same as anybody else who has left the country and is returning.
"It's advice, so everyone can decide what to do with the advice. The important thing is that the quarantine is in place."
Mr Johnson's trip caused anger among MPs, amid claims it showed there is "one rule for the Conservatives and one rule for everyone else".
The PM's spokesman said on Wednesday the Foreign Office advice on international travel remains in place, but said: "It is for individuals to make the judgements themselves."
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