New questions have been raised over how much the Bank of England knew about the “low-balling” of Libor by lenders during the financial crisis after the emergence of an email from a top financier warning officials that rates were susceptible to rigging.Paul Tucker, then deputy governor of the Bank, was told in mid-2008 by the chief treasurer of Royal Bank of Scotland that banks might not be submitting realistic rates and that “moral persuasion” might be needed to force institutions to give an accurate picture of their finances.

In an email seen by The Times, John Cummins wrote in May 2008 to Mr Tucker and the heads of treasury at all of Britain’s leading lenders outlining a series of measures to reform the now-discredited Libor-setting process that allowed traders and managers to input artificially low or high figures to improve profits or to give a false impression of their bank’s financial strength.

Mr Cummins suggested to Mr Tucker that some kind of punishment might be required to keep banks honest.

The revelation that months before the financial crisis senior bankers and policymakers were aware of potential Libor manipulation raises questions about the evidence given to MPs by Mr Tucker three years ago during a parliamentary investigation into rate-rigging after Barclays’ admission that it had submitted false figures.

Asked whether he had any knowledge of Libor being manipulated since 2008, Mr Tucker said that he had become aware of the practice only with the Barclays settlement. “We did not have any knowledge. I did not have any knowledge of this [Libor-rigging],” Mr Tucker told the Treasury select committee in 2012.

At the same hearing, Mr Tucker denied giving an instruction to Barclays to lower its Libor submissions in order to give a false impression of its financial health, a practice dubbed “low-balling”.

The denial came after the publication by Barclays of an internal memo written by Bob Diamond, in which the bank’s former chief executive described a phone call in October 2008 in which Mr Tucker was reported to have stated that “it did not always need to be the case that we appeared as high [in the Libor rate submitted] as we have recently”.

Mr Tucker told the committee that the Barclays file note gave the “wrong impression” of the conversation.

Before leaving the Bank of England in 2013, Mr Tucker had been seen as a leading candidate to succeed Lord King as governor. At present, he is working at Harvard University.

This month Tom Hayes, a former UBS and Citigroup trader, became the first person to be convicted of Libor manipulation by a jury. The British-born trader was given a 14-year prison sentence.

Mr Tucker did not reply to an email seeking comment on the latest email revelation.

In a statement, a spokeswoman for the Bank of England said: “It would be inappropriate for the Bank to comment, particularly while criminal proceedings in relation to Libor are ongoing.”

A spokesman for RBS declined to comment.