The chairman of Barclays Bank is reported to be on the brink of stepping down as the row intensifies over the Libor rate-fixing scandal.

Reports said Marcus Agius was about to leave the embattled bank, which declined to comment on the suggestions.

BBC business editor Robert Peston told BBC News: "Today Mr Agius has told his colleagues on the Barclays board that he will be going. It's not wholly unexpected. Mr Agius was first in the firing line as far as shareholders were concerned, they've been thinking for some time that Barclays might need a new chairman, he's been in the job for six years next year."

The development came as Business Secretary Vince Cable backed calls for a criminal investigation into bankers involved in the affair. The Liberal Democrat Cabinet minister said the public could not understand why the perpetrators of "what looks like a conspiracy" were allowed to "just walk away".

The potential for prosecutions arising from the scandal has been downplayed by Treasury sources who point out that there are no criminal sanctions in place for manipulating the inter-bank lending rate, or Libor.

Libor is set on a daily basis by panels of banks and used to help set "swap rates" - the borrowing rate between financial institutions. These rates in turn are used to price a vast range of products such as corporate loans and fixed-deal mortgages.

Lord Blair, a former head of Scotland Yard, said there had to be police inquiries into revelations that traders at Barclays, RBS and other banks had tried to fix the rate. He said: "Anybody, the youngest detective, would say this is conspiracy to defraud. It can mean nothing else. And therefore someone has to launch a criminal inquiry into this behaviour."

Mr Cable said the Serious Fraud Office was having "a fresh look" at the Financial Services Authority's investigation into the affair and that the public expected criminal prosecutions. "They just can't understand why people are thrown into jail for petty theft and these guys just walk away having perpetrated what looks like conspiracy," he said.

His intervention came as it emerged that taxpayer-backed RBS has sacked four staff over their alleged role in the Libor-fixing scandal. The bank declined to comment on the matter but sources said the sackings were made at the end of last year.

It is understood they were traders Paul White and Neil Danziger, investment adviser Andrew Hamilton and Tan Chi Min, who used to work for RBS in Singapore.