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The prime minister and wife Carrie took their children Wilf and Romy to the seaside in Porthminster, St Ives, during the weekend visit to the Southwest.
Downing Street insisted the "sole reason" for the flight was to transport Mr Johnson and staff back from government business.
The government plane was sent from London to Royal Naval Air Station Culdrose, near Helston, on the morning of Monday 13 June, the Sunday Mirror first reported.
On the Friday before, Mr Johnson went to the Royal Cornwall Show, where he visited cattle and sheep tents and spoke to local traders. He also met farmers with Conservative candidate Helen Hurford ahead of the 23 June Tiverton and Honiton by-election, which the Lib Dems won in a major blow to the PM.
Over the weekend, Mr Johnson was seen enjoying the sunshine on at beach at St Ives.
On Monday of the flight, he went to Southern England Farms in Hayle, where he was photographed driving a tractor, trimming a courgette and weighing broccoli, before flying back to London.
Emily Thornberry, Labour’s shadow Attorney General, accused of the PM of “treating the government’s official plane as his personal taxi service, regardless of what it costs the environment or the taxpayer”.
“It’s the act of a man drunk on power, who needs to be told he’s had enough,” she added.
A No 10 spokesperson said: "All travel decisions are made with consideration for security and time restraints.
"The PM is accompanied on government business by a delegation of staff, which is taken into consideration as part of ensuring taxpayer value for money.
"This was the sole reason for the plane being used to transport the PM and his staff back from this particular visit."
Boris Johnson was last year accused of “staggering hypocrisy” after he took a private jet back from the Cop26 climate summit to attend a private members’ club dinner in London.
His Cornwall trip also raises questions about whether the flight was justified under the ministerial code.
"Ministers must ensure that they always make efficient and cost-effective travel arrangements," the rulebook states.
"Official transport should not normally be used for travel arrangements arising from party or private business, except where this is justified on security grounds."
Family members are permitted to join ministers on the trips "provided that it is clearly in the public interest".
The same plane was used for Mr Johnson's diplomatic visit to Rwanda, Germany and Spain.
During the G7 summit in Schloss Elmau, the Prime Minister and Canada's Justin Trudeau compared the relative sizes of their jets.
Mr Johnson said he had seen "Canada Force One" on the tarmac and Mr Trudeau joked that the Prime Minister's plane was bigger.
"Very modest" was how Mr Johnson described his own jet.