Britain’s biggest anti-terrorist investigation was a £5 million overtime bonanza seized on by police as the chance to pay for Caribbean holidays, plasma televisions and nights at The Savoy.

The Times has seen e-mails circulated to officers across Thames Valley Police offering “premium rates” of pay to those “with a raging credit card habit”. Volunteers were told that night shifts, believed to be paid at £300 each, would give them time to “read a good book, take up botany or ornithology, study for your sergeant’s exam [or] work out the compound interest on a rest day’s pay”.

One message, which was marked “108 shopping days to Christmas”, sought officers for Saturday shifts and said that the payments “could buy the joy and admiration of your children on Christmas morning . . . is that not priceless?”

The internal e-mails were sent to officers across the force at the height of a big search in King’s Wood and Fennels Wood near High Wycombe, Buckinghamshire. The work was part of Operation Overt, the inquiry into an alleged terrorist plot to blow up transatlantic airliners.

Thames Valley Police said yesterday that the e-mails were “in poor taste” and that its involvement in the operation cost the force £8 million, including £4.9 million in overtime.

After the end of the airline plot trial last week, Andy Hayman, the former Scotland Yard officer in charge of special operations, disclosed in The Times that he had resisted pressure from Thames Valley Police Authority to stop the searches, which it said were too expensive.

While specialist teams searched the woodland, uniformed Thames Valley officers were required to stand guard. The e-mails seeking volunteers were sent by Sergeant David Bald to colleagues in Bletchley, Milton Keynes, Wolverton and Newport Pagnell.

Mr Bald, who signed off as “Miracle Worker” in an e-mail of August 24, 2006, added: “So there you have it. Not only would you be insuring [sic] the integrity of evidence in the most important terrorist trial in the UK for 30 years (and that is reward in itself, not to mention a great PDR [personal development record] entry) but you could also afford one of the above rewards which would give you great enjoyment and satisfaction.”

The next day he wrote: “If you’re available then please ping me an e-mail - it’ll pay off the credit card.”

Another message was circulated on September 6 and said: “For all officers (especially those scared of the dark) I now have a significant number of day shifts available on premium rates.”

On September 8 he said that the duties required “little effort, no paper-work and a restful time away from the stresses and strains of everyday life”.

The disclosure comes as the police service is increasingly concerned about its image. Ian Johnston, president of the Police Superintendents’ Association, issued a warning this week that the public was losing confidence in officers.

Home Office assessments ranked Thames Valley last year as the third-worst performing police force in Britain. A report by HM Inspectorate of Constabulary said that the diversion of Thames Valley’s resources to Operation Overt had “significantly depleted its operational capacity”.

A spokesman for the force said: “The e-mails were unacceptable but do not reflect the attitude of police officers as a whole. They were misguided and written in poor taste and recalled as soon as senior officers became aware of them.”

Hundreds of Thames Valley officers took part in Operation Overt over a six-month period. The spokesman said: “We deployed officers from their usual Thames Valley postings for 5,184 working days, at an opportunity cost of £1.4 million. This put a strain on the policing of local communities and therefore overtime at a cost of £4.9 million was used, as well as assistance from other forces at a cost of £1.9 million.”

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