After Barclays was given a record fine of £290 million pounds yesterday for trying to manipulate interest rates at which banks lend to each other, more banks could now be investigated.
Regulators in Europe, the US and Asia have said that investigations into other banks are "ongoing".
The UK's Financial Services Authority said the early signs were that Barclays had not been the only firm involved.
On Wednesday Barclay's was ordered to pay almost £300 million to settle claims that it used underhand tactics to try to rig financial markets.
A trail of emails and messages disclosed by the FSA showed how traders broke so-called Chinese Walls, which are designed to avoid conflicts of interest within financial firms, as they requested Barclays make changes to the Libor rate in a bid to boost their profits.
Chief executive Bob Diamond, who was in charge of Barclays Capital at the time the breaches occurred between 2005 and 2009, apologised and said he and three other key executives would waive their bonuses for this year.