British and Swiss regulators Have stepped up their investigation into a $2.3 billion trading loss at UBS's London arm last September with the launch of a formal probe.

The Financial Services Authority (FSA) and the Swiss Financial Market Supervisory Authority (FINMA) launched an initial joint investigation on September 16, a day after UBS unveiled the huge loss, which it blamed on a rogue trader.

FINMA said Friday they had started formal administrative enforcement proceedings against UBS under Swiss law, as well as launching a deeper investigation alongside Britain's FSA.

FINMA will assess and rule upon the adequacy of the controls that were in place to prevent and detect unauthorized trading within the investment bank, the regulator said.

UBS said Friday it would cooperate fully with its regulators.

The Swiss bank said that its ability to disclose further details about what it calls an unauthorized trading incident was limited by ongoing criminal proceedings and the regulatory investigations, but that it was still making internal changes in response to the losses.

This does not prevent us from taking further decisive action to improve our operational risk controls, UBS said.

Former UBS trader Kweku Adoboli pleaded not guilty earlier this week to charges related to the losses, and his trial in London is now due to start on Sept 3.

The enforcement investigations could also take several months.

(Reporting by Sarah White and Sudip Kar-Gupta; Editing by Hans-Juergen Peters)