The banker is the first person in Britain to plead guilty to the offence.
The person cannot be named for legal reasons.
The Serious Fraud Office (SFO) said in a statement: “A senior banker from a leading British bank pleaded guilty at Southwark Crown Court on 3 October 2014 to conspiracy to defraud in connection with manipulating Libor.
“This arises out of the Serious Fraud Office investigations into Libor manipulation. Further details cannot be given at this time for legal reasons.
“This is the first criminal conviction arising from the Serious Fraud Office’s Libor investigation.
“Eleven other individuals stand charged and await trial. The investigation into others continues.”
Two bankers have already admitted guilt in the US to Libor fraud offences.
Transactions worth trillions are based on the Libor benchmark
Libor – the London inter-bank offered rate – is the benchmark figure used as the basis for vast amounts of money globally.
Its potential for manipulation was revealed in the wake of the financial crisis.
The revelations seriously undermined the foundation of the system, which was overseen by the British Bankers’ Association.
A number of regulators around the world launched their own investigations, and they also spread to the European benchmark equivalent – Euribor.
A total of seven banks and brokerage firms have already reached settlements over rate-rigging allegations.
Last month Lloyds Banking Group said it had dismissed eight staff and was withholding bonuses due to vest, as part of a new scheme known as clawback.
Lloyds settled penalties and fines totalling £218m in the UK and US over the issue.
Regulators around the world are also examining the possible manipulation of the foreign exchange market.
In metal exchanges, silver pricing has been transferred to a new system to avoid potential manipulation, and the so-called gold-fix is due to be changed from its century-old valuation.
Collectively, the revelations and overhauls deemed necessary have undermined public and governmental trust in the whole finance sector, whose riskiest elements prompted massive bailouts amid the global crisis of 2008.