The e-mails sound casual: Dude reaching out to dude, begging for favours and offering rewards ranging from coffee to fine champagne.

But what the bankers were allegedly doing was as serious as it gets: fixing an interest rate that affects the cost of half a quadrillion dollars — that's $554 trillion — in financial contracts around the world, from mortgages to loans.

US and British investigators say the employees of Barclays Bank — and possibly those of other major international banks — clearly knew it was wrong to manipulate the London interbank office rate, known as the LIBOR, which determines the rate at which banks lend to each other and, by extension, the rate at which they lend to consumers and businesses.

One trader messaged a colleague about helping to influence the three-month LIBOR.

"As always, any help would be greatly appreciated," the trader wrote.

"I am going 90 although 91 is what I should be posting," came the reply.

The trader responded: "When I retire and write a book about this business your name will be written in golden letters."

"I would prefer this not be in any book!" came the answer.

And yet it did appear — not in a book, but in court papers that led to fines totaling $453 million against the bank. US and British officials are considering criminal charges against individuals and British investigators are probing other major banks including Citigroup in the United States, Switzerland's UBS, Britain's HSBC and Royal Bank of Scotland.

The scandal has added fuel to public anger at the banking industry, whose executives face mounting accusations of being overpaid and unethical. Shares in Barclays plummeted 15.5 per cent last Thursday as investors worried about the impact of fines and tighter regulation. The British Bankers Association confessed to being "shocked" at the accusations.

"One of the reasons London is a major international financial centre is because of the perceived emphasis on trust and integrity in the London market," said Simon Culhane, chief executive of the Chartered Institute for Securities & Investment.