Barclays Bank loans adviser Elizabeth Lockwood repeatedly betrayed her bosses after being "corrupted" and recruited to the nation-wide fraud by businessman Richard Thorpe.
The "vital" and "highly confidential" inside information about company accounts, which she passed to the former nightclub owner, enabled scores of cheques to be forged, money laundered and funds channelled through a Swiss bank.
The gang got away with nearly £2.4 million, and hardly any of it has been recovered.
Lockwood, 36, of Gresham Close, Bexley, Kent, who was found guilty with Thorpe of four counts of conspiracy to defraud between January 1991 and April 1992, was jailed for 15 months and ordered to pay £700 compensation.
Snaresbrook Crown Court judge Owen Stable QC said that to cover up her "very grave" breaches of trust during the 14-month swindle, she had told "lie upon lie upon lie".
Thorpe was sentenced to two and a half years imprisonment and ordered to pay £95,000 with a further two years in default.
Also jailed was Southampton solicitor Alexander Nicholls, who was found guilty of one count of conspiracy to defraud.
The 50-year-old lawyer of Porteous Crescent, Chandler's Ford, Eastleigh, Hants, who helped process a £417,123 cheque through a client's account, received an 18-month sentence.
A second solicitor, Leslie Burke, 51, of Broken Gate Lane, Denham, Buckinghamshire, who was convicted of attempting to obtain £235,000 by deception from a company's account, was sent to prison for a year.
The judge told Nicholls and Burke that dishonest solicitorswere "a curse to the legal profession and society at large."
A fifth defendant, businessman Andrew Boyd, 55, of James Avenue, Dagenham, Essex, also convicted of conspiracy to defraud, was jailed for eight months and ordered to pay £5,000 compensation.