Barclays boss Bob Diamond has admitted he is disappointed that the rate-rigging scandal happened on his watch and would "make sure that it cannot happen again".

The comments in a memo to staff came hours after Barclays chairman Marcus Agius resigned and announced an internal review into the bank's "flawed" practices.

Despite mounting calls for his own departure, Mr Diamond showed no signs of stepping down soon, telling employees in a lengthy letter: "I am committed to ensuring the recommendations of this review are implemented in full."

The memo emerged after the Serious Fraud Office said it hoped to decide within a month on whether a criminal prosecution in relation to the rate-fixing was appropriate.

Barclays has been at the centre of a gathering storm over banking ethics after it was last week fined £290 million by UK and US regulators for manipulating the Libor, the rate at which banks lend to each other.

In the letter, Mr Diamond, who will appear before the Treasury Select Committee on Wednesday, said it had been "an incredibly tough period" for staff given the "nature and volume of negative comment" against Barclays.

He said: "I understand why the reaction has been severe. No one is more sorry, disappointed and angry about these events than I am." He added: "I am disappointed because many of these behaviours happened on my watch. It is my responsibility to make sure that it cannot happen again."

Turning to taking action against those involved, Mr Diamond said that an internal disciplinary process, which began some time ago, will be completed swiftly.

There have been mounting calls for criminal prosecutions to be brought against those responsible for fixing the Libor for their own personal benefit.

The Chancellor is expected to reveal further details on an inquiry into Libor-setting and strengthening criminal sanction powers. The review is reportedly to be headed by an independent figure to ensure a speedy response to the issue, resulting in amendments to the Financial Services Bill this summer.