The former chief executive of the Serious Fraud Office received £422,000 in pension and severance payments without proper authorisation, the Whitehall spending watchdog has said.

The National Audit Office ruled that the payments made to Phillippa Williamson when she took voluntary redundancy earlier this year were "irregular".

As a result, it said it was "qualifying" the SFO's annual accounts for 2011-12.

The head of the NAO, Amyas Morse, said: "By failing to seek approval from the Cabinet Office and the Treasury, the Serious Fraud Office entered into an agreement which forced it to make irregular payments. While positive steps have been taken by the incoming director, I have qualified the organisation's accounts."

Ms Williamson left the SFO last April. However, the NAO said there was no evidence that "due process" was followed in instigating voluntary redundancy - such as determining whether any alternative position within the Civil Service had been sought.

On leaving, her severance agreement provided for £407,000 to be paid to MyCSP, which administers the principal Civil Service pension scheme, to cover the additional pension costs arising from her early departure. The SFO should have gained approval from the Cabinet Office for the payment, but the NAO said there was no evidence that it did.

Ms Williamson also received a "special severance payment" of £15,000 which should have received advance approval from the Treasury as it was "in excess of contractual amounts" - but again the NAO said there was no evidence that such approval was sought.

When the new SFO director, David Green, assumed office shortly after Ms Williamson left, he sought legal advice which confirmed that her agreement with the SFO was legally binding and that the SFO was legally obliged to make the payments.

Nevertheless, he commissioned an independent review into the matter, with the results being shared with the Attorney General's Office, as well as taking steps to ensure that all future exit packages were appropriately reviewed.

The chairwoman of the Commons Public Accounts Committee, Margaret Hodge, said she was "astonished" that the SFO had agreed to the severance payment to Ms Williamson. She added: "My committee will want to see an immediate stop to unacceptable payments of this kind and for the SFO to ensure all future packages for those leaving the organisation are properly approved."

The Press Association, photo by