MI5 is looking for an historian to write a warts-and-all account of the service, to mark its centenary.

The successful candidate will be required to become a full secret service member for the duration of the work and sign the Official Secrets Act.

Many books have been written about MI5 - usually with the help of past agents - but no writer has ever been allowed to comb the estimated 500,000 secret files that cover everything from anti-spy operations in the World Wars to undercover work in Northern Ireland.

Former agent David Shayler is probably not on the author shortlist

A spokesman for the Home Office told BBC News Online the book will be released in the centenary year of 2009 and will chart the role the security service has played in influencing government policy since its founding in 1909.

The writer will be given security clearance of the highest level and will be free to make judgments about MI5's mistakes, as well as highlight its successes.

They will also have access to all serving officers and, when they agree, former officers.

In confidence

One such agent, David Shayler, caused a storm when in 1997 he made a string of newspaper revelations about the secret services, including an alleged plot to kill Libyan leader Colonel Gaddafi.

Because he did not have clearance, he was accused of breaching the Official Secrets Act and his trial is due to take place at the Old Bailey later this year.

According to the Times, agency chief Sir Stephen Lander has written in confidence to a select number of historians inviting them to apply for the job.

It added that the author would be employed part-time for six years and could expect an annual salary of between �30,000 and �40,000.

All royalties from the book, expected to be a best seller, will go to the Treasury.