The money is paid from a fund called the Public Duty Costs Allowance, which is designed to help former prime ministers with the cost of their remaining public duties, such as answering letters or attending events.

But unlike MPs' standard expenses, the public is not told how the money is spent.
Receipts are not published, and it is not clear whether the former prime ministers even need to submit receipts to claim the cash.

Walsall MP Valerie Vaz (Lab Walsall South) raised concerns in the House of Commons earlier this month, when she asked: "May we have a statement on the actual work that these former Prime Ministers have done and whether they have had to provide receipts?"

But she still hasn't received an answer.

Speaking again in the Commons this week, she said: "On 3 November I raised an issue in respect of the public duty costs allowance. Uncharacteristically, I have not yet received a response from the Leader of the House as to why £1.7 million has been paid to former Prime Ministers.

"That money has obviously gone through the Government's books in some way, so may we have a list of exactly what duties were performed, and whether invoices and receipts had to be provided?"

Sir George Young, leader of the Commons, told her: "I am sorry if the hon. Lady has not received any information that she is entitled to. I will chase this matter up the moment these business questions finish, and make sure that she gets an answer from the appropriate Minister."

In a written answer in the Commons in October, Ministers revealed how much had been claimed by former Prime Ministers during the past five years. The figures do not include money paid out so far in the current financial year.