This is after it was revealed that staff at Barclays were found to be rigging interest rates for profit, a practice which is illegal and has an impact on customers further on down the line.
Diamond denied any knowledge of malpractice in an open letter to Andrew Tyrie MP, the chairman of the Commons Treasury Committee, a claim that he says the investigating authorities concluded.
He wrote, "When the trader conduct was first discovered by more senior management, steps were immediately taken to stop it, and it was reported to the authorities."
Mr Diamond added that Barclays was now "completing a review of employee conduct for all those involved".
Despite his letter, many MPs are less than convinced. Many MPs have suggested Mr Diamond should resign and former Liberal Democrat leader Paddy Ashdown told BBC One's Question Time his position was now "untenable".
Today’s letter comes after Barclays were slammed by the Prime Minister yesterday. Mr Cameron said in a speech yesterday, "The whole management team [at Barclays] have got some serious questions to answer. Let them answer those questions first. Who was responsible? Who was going to take responsibility? How are they being held accountable?"
Labour leader Ed Miliband added that those found guilty of wrongdoing "should face the full force of the law".