Mr Blair says use of the royal flight aircraft was justified because he met Italian premier Romano Prodi before starting a two week holiday.

Mr Blair, his wife Cherie, their three children and his mother-in-law travelled on the Royal Air Force jet on Saturday.

His Downing Street office said it was proper that Blair should not pay for his seat.

A spokesman added that fares for the rest of his family had been paid for.

The Sun newspaper attacked Blair for taking a "freebie" at the expense of British tax-payers.

And it pointed out that rather than having to join other travellers at a civilian airport, the Blairs had travelled tax-free from RAF Northolt.

The Mirror, which normally supports Mr Blair, said the prime minister appeared to be developing "a worrying taste for the trappings of power".

'Blair Force One'

The Queen and Tony Blair: Said to be in disagreement over plans for the plane

The prime minister was also criticised over reports alleging that he had blocked cost-cutting proposals to share a plane with the Queen, because they cannot agree on the size of the craft.

The Queen is reported to favour a 19-seater executive jet.

But Mr Blair is said to prefer a much larger Airbus to accommodate his entourage of aides, security staff and journalists.

The Sun has dubbed the plane "Blair Force One", drawing on similarities with the aircraft used by the US president.