The Luxembourg-based General Court ordered the European Commission to get more details on certain elements of the mechanism which pays utilities to make electricity available at short notice.

"The Commission's investigation will focus, in particular, on the participation of energy consumers offering to reduce their electricity consumption in times of supply disequilibrium in the electricity market," the EU executive said.

Centrica, SSE and Drax had won contracts under the scheme.

The British government has said that in the event of a so-called hard Brexit, where Britain exits the bloc without further agreements, British legislation concerning state aid - under which capacity auctions take place - comes into law.

(Reporting by Foo Yun Chee)