Barclays is immune from a potential €690 million (£573 million) penalty after the bank blew the whistle on the Euribor cartel.
The Euribor is the euro area equivalent of the London interbank offered rate (Libor).
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The man was taken early this morning to a south London police station where he remains in custody.
The Chancellor faces tough talks to build a Government while also dealing with the challenge from a resurgent far right.
Watch John McDonnell's speech at Labour Party conference 2017 live as he speaks to Brighton the audience from 12.15pm.