The Duke and Duchess of Edinburgh pocketed rental income from a royal charity by sub-letting a stable block at their Surrey home that they leased from the Crown Estate.

Prince Edward and wife Sophie earned an undisclosed sum after renting out a unit in the grounds of their Bagshot Park mansion to the Royal Collection Trust.

The RCT is the registered charity that cares for the Royal Collection, one of the world's greatest troves of art and antiquities that is held in trust by the sovereign on behalf of the nation.

Sources insisted last night that the arrangement was only for a short period of time and is not unusual, since the Royal Household also charges rent to the same charity for expenses such as office space.

But while the move was entirely within the terms of the Edinburghs' lease, the arrangement is likely to be carefully examined by MPs as part of a wider investigation into the Royal Family's property dealings amid public concern that they have been benefiting from favourable deals.

A National Audit Office (NAO) report published yesterday, which will form the basis of their enquiries, revealed that shamed Andrew Mountbatten-Windsor also received an undisclosed rental income from sub-letting three cottages on his Royal Lodge estate at Windsor that he leased from the Crown Estate.

It is understood the property was rented to staff and/or retired staff and the amount was apparently only enough to cover running costs. 

This has not been independently verified as the NAO said it had no details of what monies were received by the former Duke of York. Whatever the amount, it went back to Mr Mountbatten-Windsor rather than the Crown Estate, which pays its profits to the Treasury.

The report also disclosed that his daughters, Princess Beatrice and Princess Eugenie, have not personally paid rent for their royal homes in almost two decades, with the costs secretly met by their grandmother, Queen Elizabeth, and more recently their uncle, the King.

It can now be revealed that Edward and Sophie have benefited from a similar arrangement to Andrew as regards their Grade II-listed, 120-room mansion 11 miles south of Windsor Castle.

The property was first leased to Edward in 1998 via a company, Eclipse Nominees Limited (Bagshot Park), which saw him pay £5 million up front.

This means he has only ever had to pay a 'peppercorn rent' since - a nominal amount each year for legal purposes. The late Queen's son has also spent £1.38 million on refurbishment and has a 150-year lease on the property until 2157.

As part of the lease he is entitled to sub-let parts of the estate which, according to the NAO, 'allows the leaseholder to generate income for themselves, which is not returned to The Crown Estate'. 

The report revealed that the Edinburghs invested a 'significant' sum doing up an old stable block which was rented out to a third party until six years ago.

The lease allows the stables to be used as offices, research and development, for the stabling of horses or residential accommodation. Previous reports suggest he had earned up to £130,000 a year from the scheme.

What was not known, until now, is that another unit in the block was also rented out for a 'short period' of time to the RCT. A trust spokesman said: 'We do not disclose details of storage locations or arrangements.'

Buckingham Palace has said it is 'grateful' to the NAO for the report which 'is in line with the Royal Household's commitment to transparency'.

The Palace also said that all financial arrangements are kept under review, which includes the agreement for the King to fund his nieces' rent. 

Although no decision has yet been made about whether he will continue to do so, it is understood it will be looked at in the next 12 months.