The Opel trustees are set to make a decision on Thursday about the future of General Motors' European business should GM decide to sell it to an investor, a source close to the matter told Reuters.

GM has sent a negotiator on Opel back to Berlin after a watershed two-day GM board meeting, people familiar with the proceedings had said on Wednesday.

It was not immediately clear what action the GM board had decided on for Opel after spending the past month weighing the merits of selling the European unit against the cost of keeping it.

The Opel trustees oversee a majority stake in Opel and have to approve any decision.

GM Europe declined to comment on the news.

(Reporting by Angelika Gruber)