Boris Johnson's father has been criticised for travelling to Greece during the coronavirus lockdown.
Stanley Johnson shared a number of pictures on his Instagram account on Wednesday, showing him arriving in Athens and at an airport in a mask.
He told the Daily Mail he was in the country "on essential business" to ensure a property he rents out was "Covid-proof" before holidays restart.
But the former Tory MEP has come under fire for breaking lockdown rules.
Liberal Democrat MP Jamie Stone said the incident "stinks of one rule for them and another rule for the rest of us".
When asked about his father's behaviour, the prime minister told LBC: "I think you really ought to raise that with him."
The current guidance on air travel from the UK Foreign Office advises against "all but essential international travel" because of the virus.
Anyone who then returns to the UK has to isolate for 14 days, under the government's existing rules.
Liberal Democrat MP Alistair Carmichael raised concerns in the Commons, saying the prime minister could "explain his views on the fact that apparently his own father has jetted off in defiance of the guidance to Greece".
Referencing reasons given by Boris Johnson's most senior aide, Dominic Cummings, when he was accused of breaking the lockdown rules in March, Mr Carmichael added: "Maybe, I don't know, he just needed an eye test or something like that, but I think we would all welcome an explanation."
Answering for the government, Commons leader Jacob Rees-Mogg said: "I seem to remember somewhere in the Bible that the sins of the father will be visited on the son, but I don't remember it ever being the other way round.
"I think that the honourable gentleman is really fishing desperately to try and make any criticism of the PM."
People arriving in England from more than 50 countries including France, Spain, Germany and Italy will no longer need to quarantine from 10 July, the Department for Transport has confirmed.
The Scottish and Welsh government have yet to announce any changes to regulations, while in Northern Ireland, quarantine remains in place for travellers arriving from outside the UK and the Republic of Ireland.