British bank Barclays will pay at least $450 million to U.S. and British authorities to settle a probe into manipulation of the key interbank lending rate known as Libor.

Regulators have been investigating allegations that several banks, including Barclays, manipulated the London Interbank Lending Rate (Libor), which underpins trillions of dollars of derivatives contracts worldwide and is also widely used as a reference rate for corporate lending.

Barclays regularly reported borrowing rates lower than the rates it was actually paying during the financial crisis, in order to mask its distress, according to a statement from the U.S. Commodity Futures Trading Commission on Wednesday.

Damning emails that regulators released on Wednesday make clear that traders and the "submitters" tasked with reporting daily rates worked together for years to make the rates submitted suit the traders' and the bank's purposes.

In some cases, submitters set themselves reminders on their calendars to submit low rates on certain dates, according to the emails. In others, traders expressed overwhelming gratitude for low submissions that protected them from losses.

The U.S. CFTC said Barclays attempted to manipulated Libor submissions "sometimes on a daily basis" over a four-year period starting in 2005. The CFTC ordered the bank to pay a $200 million penalty, saying it was the largest civil monetary penalty it has ever imposed.

Barclays also settled with the U.S. Department of Justice and the UK's Financial Services Authority and will pay fines of $160 million and $92.8 million, respectively.

The Department of Justice said Barclays was the first bank being probed "to provide extensive and meaningful cooperation to the government," adding that the bank's assistance had aided its criminal investigation.

In March the bank said it was engaged in a possible resolution with regulators looking into potential enforcement proceedings.

As well as the FSA and CFTC, other authorities probing Libor manipulation include the European Commission and Japan's Financial Services Authority.

Other banks involved in the probe include Citigroup, HSBC, Royal Bank of Scotland and UBS.

Several banks have suspended traders over the investigations. No criminal charges have been filed.

Libor is the benchmark for about $360 trillion worth of financial contracts worldwide. A daily poll asks banks at what rate they think they will be able to borrow money from one another in 10 major currencies and for 15 borrowing periods, ranging from overnight loans to 12 months.

Thomson Reuters Corp is the British Bankers' Association's official agent for the daily calculation and publishing of the Libor rates. A spokesman for the company was not immediately available for comment.

As the credit crisis took hold in 2008, allegations started mounting that Libor no longer reflected banks' real borrowing costs, and authorities began examining whether traders tried to influence whether the rate went up or down to profit on bets on its future direction.