More banks could be caught in the scandal which saw Barclays pay £290 million to settle claims that it used underhand tactics to try to rig financial markets.
The Financial Services Authority (FSA) disclosed that it had a number of other investigations under way in the wake of the allegations that Barclays manipulated the rates at which banks lend to each other.
"We have a number of investigations that are ongoing," Tracey McDermott, the FSA's acting director of enforcement and financial crime, told BBC's Newsnight. "Obviously we need to look at each case on its own particular facts but the initial indications are that Barclays was not the only firm that was involved in this."
The chairman of the Commons Treasury Committee, Andrew Tyrie, said they would now be summoning Barclays chief executive Bob Diamond to account for what had happened. "Banks were clearly acting in concert. I fear it's not going to be the end of the story, that we are going to find that other banks have been involved," he said.
Meanwhile, Labour leader Ed Miliband is expected to call for a criminal investigation into the scandal. Speaking to the union Unite, Mr Miliband is due to say: "This cannot be about a slap on the wrist. When ordinary people break the law, they face charges, prosecution and punishment. The same should happen here."
The penalties from UK and US regulators, including a record £59.5 million fine from the Financial Services Authority (FSA), followed claims Barclays manipulated the Libor and Euribor interbank lending rates. In the depths of the financial crisis, Barclays gave false information about the interest rates it had to pay to borrow money in an effort to paint a false picture of its health to markets.
Mr 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.
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.
In one request for a change to the Libor rate, a trader said: "Coffees will be coming your way either way, just to say thank you for your help in the past few weeks". To which the Barclays submitter responded: "Done, for you big boy."
Former Barclays chief executive Sir Martin Taylor said the bank should explain who knew what about the practices. He told BBC Radio 4's Today programme: "The question of how high up knowledge of this goes is something only Barclays can answer. I think they should answer. Somebody at senior level somewhere will certainly have known. I can't believe Barclays haven't identified who that is."