Photo: © 2009 AP via AAP/CHRIS YOUNG

A News Of The World journalist posing as a businessman secretly filmed Sarah Ferguson accepting a handsome down payment on the deal.

In return for the full payment – close to $870,000 dollars – she promised the undercover journalist access to the Prince Andrew, who is now Britain's special representative for international trade and investment. 

A cash-strapped Ferguson says she very deeply regrets what she did.

"It is true that my financial situation is under stress, however, that is no excuse for a serious lapse in judgment and I am very sorry that this has happened," she said in a statement.

The paper says the prince knew nothing of the deal, despite its secret footage showing her saying he did.

Its royal editor, Robert Jobson, says she sold "not only her ex-husband but the country, the monarchy and herself down the river".

"People are going to doubt the integrity of what he is doing - which is ridiculous and wrong - because of the behaviour of his ex-wife."