Tom Hayes, the trader jailed last year for his role in attempts to manipulate Libor, has written to the head of the Serious Fraud Office over fresh claims that the Bank of England was aware of how the key benchmark was “lowballed” in the run-up to the financial crisis.

Hayes, a former UBS and Citigroup trader who is serving 11 years for conspiracy to defraud, has maintained that many banks submitted Libor rates that reflected the market and also suited their commercial positions, with the tacit blessing of regulators.

The daily rate is meant to measure how much banks expect to pay to borrow from each other, and is used to set the rates in $300 trillion-worth of loans around the world.

Hayes has written to David Green QC, head of the SFO, after an email emerged in a civil lawsuit suggesting the Bank of England knew banks were stretching the truth of their Libor submissions, known as “lowballing”, to downplay the stress in the inter-bank lending market.

A civil case against Lloyds heard last week that senior bankers from various institutions discussed this practice in 2007 with Paul Tucker, a former Bank of England deputy governor, with the firms agreeing that their submissions "do not reflect where we can borrow decent size and as such there was a case for us fixing Libors considerably higher”.