The Bank of England suspects traders tried to rig the price of one of its bond buybacks and has asked the Financial Conduct Authority (FCA) to investigate.
Executive director for markets Paul Fisher gave evidence to MPs about the central bank's quantitative easing stimulus programme and he said some of the movements in the price of the 2017 bond auction were suspicious.
The Bank of England has bought £375 billion worth of bonds in an attempt to release money to banks and the wider economy. With banks receiving more funds, they can distribute loans to the small and medium-sized businesses that need it.
However, the bank found some unusual rises in some auctions in recent years when trying to buy government bonds, which are known as gilts.
Mr Fisher told MPs: "It was very obvious to us what was happening, because one gilt was moving in the opposite direction to other gilts in the market, so there was a clear signal."
By Claire Archer