The stolen cheques were then, the company believes, paid into possibly 20 different banks where fictitious accounts had been opened under the name of AAH Pharmaceuticals Limited, the otherwise legitimate subsidiary that pays cheques into its local Lloydsbank in Runcorn, Cheshire.
An unspecified amount of money is also understood to have been withdrawn from some of the accounts at the banks, most of which were sited close to the M6 motorway from Birmingham, through Manchester and up to Blackpool. A police spokesman said that a 32-year-old man from Manchester was helping with the inquiry. There were unconfirmed reports last night that a picture of one person paying cheques into a bogus account in Leeds had been captured on the bank's security cameras.
John Padovan, chairman of AAH - who also yesterday surprised the City with a warning about losses in AAH's environment services division - said there was "no evidence that any of our staff have participated in this".
He also said he had no criticism of the Royal Mail.
The mailbag was one of many containing a total of £80m of cheques. "We will be reviewing our banking system and use of postal services. We may look to see how a mailbag is received into the building," Mr Padovan said.
David Taylor, head of AAH Pharmaceuticals at Runcorn, said it was not yet clear how the cheques had fallen into the wrong hands.
Meanwhile, analysts were left wondering whether AAH would continue with its environmental services businesses. The company said the operation would lose £3m in the year to 31 March, compared with a profit of £3.9m in 1993/4.
Mr Padovan also warned that profits from distribution services would be substantially below last year's £2.3m, but that the mainstay healthcare businesses continued to perform soundly.
AAH shares fell by 21p to 282p yesterday, virtually half the peak 544p of a year ago.