Revealed: How the Speaker has his own Commons credit card to entertain political friends

It means he never has to dip his hands into his own pocket when on official business. If he wants to buy a meal or a round of drinks, all he has to do is to charge it to his personalised card.

It works like most others from Visa or American Express - with one vital difference. When the monthly bill arrives, the whole amount is paid off by the taxpayer, not by Mr Martin.

All he has to do is submit the receipts to the Commons authorities.

It is the latest in a wide range of perks available to Mr Martin which have been revealed.

He is already under fire for his wife Mary's £4,000 taxi bill for shopping trips with her Commons cleaner.

Together with his attempt to block the release of more details of MPs' expenses, it has led to growing calls for him to step down as Speaker.

Insiders say Mr Martin, 62, has made frequent use of the Commons credit card since he became Speaker eight years ago.

A source said: "It is very handy when he is on a foreign trip. If he and his wife want to entertain fellow politicians and their partners at an overseas conference, he can use the card to pay the bill.

"It is very convenient and there is nothing improper about it. There is no reason why the Speaker should pay the money himself if he is on official duty."

The card can also be used to pay hotel bills, flights and emergency items.

He can use it when he is in his Glasgow constituency but only if the expenditure is connected with his duties as Speaker, not as a constituency MP.

On top of his £137,000 salary, the Speaker benefits from a grace-and-favour apartment at Westminster, a gold-plated pension and a £45,000-a-year entertainment budget.

He has claimed £75,000 in his second-home expenses allowance, even though he has no mortgage on his Glasgow home.

It was disclosed that he used air miles gained on official Parliamentary business to pay for flights to London for other members of his family. And more than £50,000 has been spent on flights for him and Mary to make foreign visits.

Mr Martin spent more than £20,000 of public funds paying top libel lawyers Carter-Ruck for advice.

Taxpayers also fund the cost of limousines which ferry Mr and Mrs Martin to the airport from their homes in London and Glasgow.

A spokeswoman said last night: "The Speaker, like all officers of the House, incurs expenditure in pursuit of his duties.

"He has a facility for making payments in carrying out his duties. All expenditure has to be fully authorised and audited."