A powerful City lawyer was facing jail yesterday after being found guilty of conning one the world's richest banks out of millions of pounds.

Kate Johns, 39, deputy head of Bank of Tokyo-Mitsubishi's legal department, repeatedly cajoled colleagues into ignoring procedure and approving six- and seven-figure credit letters. Southwark Crown Court in London heard that the massive transfers ended up in the coffers of the struggling Indonesian carrier Air Efata, which owned just three planes.

In return, she received pay-offs totalling £1.95m, more than £1.1m of which she used to clear the mortgage on her luxury town house. She also splashed out £40,000 of her "ill-gotten gains" on a pair of earrings to go with a £35,000 ring bought for her by the airline's grateful boss, Frank Taira Supit.

Investigators discovered she had received another £1m from him before the scam, which she claimed was for legal advice. Johns used the money to fund a lavish lifestyle of shopping trips, personal grooming and breast surgery, but her massive betrayal of trust failed to rescue the airline, which was later grounded. Mr Taira Supit, a 58-year-old former lawyer and Harvard graduate, was so devastated that he hanged himself.

Johns, from Camden in north London, told the court that, far from being a criminal, she had simply been doing the bidding of one of her bosses to protect the bank's interests. But the jury were not convinced. She was convicted of 12 offences committed between May and October 2006.

Five offences concerned using letters of credit to obtain money transfers totalling $10.9m (£6.67m) by deception, while the remainder involved the acquisition of £588,373 of criminal property.

The court heard that the £150,000-a-year lawyer, who was regarded by her colleagues as "well-respected, hard-working, and ambitious", joined the bank's London office 11 years ago. But three years ago, she sacrificed her increasingly lucrative career in favour of a criminal one.

Triggered by what police believe was a perfectly innocent request from Mr Taira Supit for help in financing his fledgling airline, she set about conning colleagues into sanctioning letters of credit. Elizabeth Marsh QC, prosecuting, said that, while many of her co-workers felt uneasy about breaching procedures, Johns claimed she was merely doing a "special job" for Yoshinobu Onishi, chief executive of the bank's European division.

As a result of the first three fraudulent letters of credit, Mr Taira Supit received $900,000 which kept the airline afloat for just two weeks. Johns then decided to up the stakes, drawing up two further letters of credit worth $5m each. When that did not work, the airline folded and she was finally arrested in January last year.

The case was adjourned for pre-sentence reports.