By Alan Baldwin

DUBAI, Dec 3 (Reuters) - Formula One could expand to 12 teams now that General Motors' brand Cadillac has been accepted as the 11th from 2026, according to FIA president Mohammed Ben Sulayem.

Formula One has always had provision for 12 in the rules but there was strong opposition from commercial rights holder Liberty Media and existing teams to going beyond 10 before a deal was reached.

"Why not?," Ben Sulayem told Reuters at Sunday's Qatar Grand Prix when asked if he would like to see the final slot filled.

"It's about doing the right thing. So why do we have an option of 12 if we are going to say no, no, no?

"With me it is very clear it is a win for everyone with the 11th team."

Cadillac last week announced an agreement in principle with Formula One while General Motors registered with the governing FIA as a power unit manufacturer to become a full works outfit by the end of the decade.

Formula One had said in January it doubted the bid, originally presented as Andretti, would be competitive or add value but relented after the original approach was re-positioned as a manufacturer-led one and Michael Andretti stepped back.

An investigation opened by the U.S. House of Representatives Judiciary Committee into possible "anticompetitive conduct" also changed the landscape.

Ben Sulayem hailed the agreement as very important for the sport and said he had been "sent to hell" and back after the FIA approved the Andretti bid last year and passed it on to Formula One for consideration.

U.S. PROBE

Ben Sulayem said the U.S. inquiry into the sport had an effect on the outcome.

"I had a meeting with them and I was questioned. I have nothing to hide. I'm an elected president, you know ... based on governance and democracy and transparency. So we did what the FIA did. And I am proud of what the team did," he said.

Ben Sulayem said the bid was always about quality rather than numbers - getting General Motors fully onboard as a manufacturer and not about Andretti. Formula One chief executive Stefano Domenicali had agreed with that.

"He said 'we need an OEM (manufacturer), not just an extra team'," the Emirati said. "So they disappeared for a few months and they came back with an OEM."

Ben Sulayem said Formula One had then raised the power unit as an issue. "So they came up with a power unit. They ticked the boxes there. And we couldn't say any more no to them."

Andretti was the sole applicant sent forward for commercial discussions with Formula One from four that made the second stage of the process last year.

Failed applications included New Zealand-based Rodin Cars, who had committed to reserving one seat for a female driver, and a Hitech team backed by Kazakh billionaire businessman Vladimir Kim.

Hitech boss Oliver Oakes is now principal of the Renault-owned Alpine F1 team.

(Reporting by Alan Baldwin, Editing by Alison Williams)

((alan.baldwin@thomsonreuters.com; +442075427933;))