Queen Mother was expected to die before 1975

Whitehall officials estimating the level of the Civil List in 1970 believed the Queen Mother would be dead in five years, secret files released today reveal.

They proposed basing future calculations on the presumption that the Queen Mother would die in 1975, and were also widely inaccurate in their assessment of when Prince Edward would marry, thereby increasing his entitlement to Civil List grants. With the Queen Mother living 27 years longer than expected, they underestimated the cost of supporting her by £2.5m.

Edward Heath, the Prime Minister at the time, knew his Government was under pressure to increase the Civil List, which had been fixed since 1952, but was devalued by inflation and about to go into the red. The Lord Chamberlain had written to Mr Heath saying that rising costs were a matter of urgency and asking for a personal meeting.

As a result, the Prime Minister commissioned a secret report by three senior civil servants into the most appropriate method and level of funding for the royals. They proposed increasing annual payments to all royals on the list and doubling the Queen Mother's from £70,000 to £140,000. This was part of a proposed system that allowed the payments to be changed as necessary, so the Queen Mother's annuity could be switched to younger royals on her death.

A note from Robert Armstrong, the finance, home and general under secretary, made it clear that officials thought the change would happen sooner rather than later.

"Any calculation has to take account of such imponderables as the expectation of life of the Queen Mother and the matrimonial prospects of Prince Andrew and Edward, but it might be worth ... assuming that the Duke of Gloucester dies in 1972, the Queen Mother in 1975 and Prince Andrew and Prince Edward each get married when they are 25," he wrote.

In the event, Prince Edward married Sophie Rhys-Jones in 1999, when he was 35. The prediction was more accurate for Prince Andrew, who was 26 when he married in 1986.

When the new Civil List settlement passed Parliament in 1972, the Queen Mother was given a lower than proposed annuity of £95,000, equivalent to £2,585,000 over the 27 years between 1975 and 2002.

The 1972 Act set the level of the Civil List for 10 years and the documents, released by the public records office, show officials and politicians considered that an annual vote on the level could harm the monarchy.

Minutes of a meeting between the Prime Minister, his Chancellor and senior civil servants showed they thought an annual vote "raised all the difficulties created by an annual opportunity for MPs and the public to raise the questions not just on the finances of the monarchy but on the activities of the royal family".