The 16-acre estate, at Nether Lypiatt near Stroud, is believed to be possibly worth more than �10m.

Savills estate agents say the house will be marketed from the end of April or early May, but declined to comment on estimates of its value.

The royal couple, who bought the manor in 1980, also have a "grace and favour" apartment at Kensington Palace.

Down-sizing

The Queen, Prince Michael's cousin, agreed in December 2002 to pay from her private funds the �120,000-a-year rent for the five-bedroomed apartment for up to seven years.

This followed demands by MPs on the Commons Public Accounts Committee that a commercial rent should be paid for the property.

Buckingham Palace said at the time that the rent was being paid in recognition of the prince and princess's unpaid work in support of the monarchy and various charities.

It appears that the prince, 62, and his 60-year-old Bohemian-born wife may now be looking to the future by downsizing and providing a cash flow for their retirement.

Their children, Lord Frederick Windsor, 25, and Lady Gabriella Windsor, 23, are understood to spend little time at Nether Lypiatt Manor and are keen to be in London.

The early-18th century country house was bought by the royal couple in 1980, for about �300,000.

The value has since soared, although a �10m sale may be hard to achieve.