Britain's top anti-terrorist officer - former Norfolk chief constable Andy Hayman - has resigned, it was announced this afternoon.Mr Hayman was facing an investigation over expenses claims and foreign trips with a female police sergeant.

Britain's top anti-terrorist officer - former Norfolk chief constable Andy Hayman - has resigned, it was announced this afternoon.

Mr Hayman was facing an investigation over expenses claims and foreign trips with a female police sergeant.

The Metropolitan Police Authority confirmed last week that a report was being prepared for consideration by the force's professional standards subcommittee.

It was believed Mr Hayman, who left Norfolk in 2005 to take up the high-profile job as the Met's assistant commissioner, was to be asked to explain at least £15,000 in expenses including claims for large amounts of alcohol and entertaining.

Mr Hayman, whose duties included leading the fight against Islamist terrorism and protecting the Royal family, said of his retirement: “It has been a great honour and privilege to lead Specialist Operations in its challenging work protecting this country from the palpable terrorist threat we face.

"This role requires total commitment in both time and effort and has a considerable impact on your personal life, your family and friends. It also puts you in the spotlight, often in ways that are very hurtful. Recent weeks have seen a series of leaks and unfounded accusations about me, which I have and will continue to refute strongly.”

"However, these events take their toll on you personally and I feel now is the right time for me to step aside and for a new person to take over as Assistant Commissioner Specialist Operations and Head of the ACPO Terrorism and Allied Matters business area. I wish my successor and all my colleagues every success in their continued efforts to defend the United Kingdom."

Metropolitan Police Authority chairman Len Duvall said: “Andy Hayman has made enormous strides in building the new national arrangements for counter terrorism, as well as modernising the Specialist Operations Directorate in the Met. He has achieved this during an extremely demanding and difficult period and we all have a lot to thank him for.”

Metropolitan Police Commissioner, Sir Ian Blair said: “Andy Hayman can be very proud of the achievements and successes during his time in charge of Specialist Operations. In this year alone, 37 people have been convicted in terrorist related cases following investigation by the Met. He has also made a significant contribution to the development of policing and the service during his career. I fully understand his decision to leave at this time and wish him well for the future.”