U.K. prosecutors are poised to arrest former traders and rate setters at UBS AG, Royal Bank of Scotland Group Plc and Barclays Plc within a month for questioning over their role in the Libor scandal, a person with knowledge of the probe said.

The arrests will be made by police under the direction of prosecutors at the Serious Fraud Office within the next month, said the person, who declined to be identified because the matter isn't public. Arrests in the U.K. are made at an early stage of the investigation, allowing police and prosecutors to question people under caution and may not lead to charges.

The SFO has 40 people working on the probe into manipulation of the London interbank bank offered rate, a benchmark for financial products valued at $360 trillion worldwide, and has involved the City of London Police, said David Green, the agency's director.