Ministers are facing questions over the hundreds of journeys they have taken on the Queen's Flight.

Critics said Tony Blair and other ministers had used the planes like a "private taxi service".

The prime minister alone has used the RAF aircraft more than 670 times since 1997 and cabinet colleagues, including environment secretary Margaret Beckett, have also been significant users.

Blair's trips on the private jet include family holidays, while Beckett uses the plane for short-haul trips to Brussels, sometimes via her constituency.

The Conservatives said there were now serious questions over "abuse" of the Queen's Flight.

Shadow transport secretary Chris Grayling said: "I think most people will be astonished at the way ministers seem to be using the Queen's Flight as a private taxi service.

"Of course, ministers will always need to use official aircraft on occasions, but these figures create the sense that the system is being abused."

The Liberal Democrats have also demanded the reasons for all the flights be made public.

They say the prime minister has an obligation to the public purse and the environment when considering his travel arrangements.

Treasury spokesman Vince Cable said: "To ensure that propriety and the ministerial code have been observed, full disclosure of the reasons for the flights should come from Number 10.

"In the code on ministerial travel the prime minister himself states that ministers are responsible for 'justifying their actions
and decisions to parliament' and to ensure that 'arrangements could be defended in public'".

Details of ministerial use of the Royal Flight were published by the Ministry of Defence under a freedom of information request.

But a spokesman for the prime minister pointed out that Margaret Thatcher and John Major used RAF flights for their holiday travel when they were in office, and Blair stopped this practice in 2000.

He added that Labour had spent less every year on ministerial travel since coming to office than the Conservative administration did in 1996.

And it was cheaper to transport ministers and officials to a meeting on a full RAF plane than on a standard commercial flight, the spokesman said.