Former French foreign minister Laurent Fabius will step down as president of COP21, the U.N.'s climate forum, after being appointed head of France's constitutional court, according to a resignation letter seen by Agence France Presse.

Fabius wrote on Monday to President Francois Hollande "to tender (his) resignation" as head of COP21, during which he helped to steer the troubled U.N. climate talks to a successful conclusion in Paris last December.

Fabius, 69, had expected to stay in the one-year post until November, but drew flak in the French political arena for seeking to hold on to two high-profile jobs at the same time.

He played a key role in sealing a historic deal under which the 195 UNFCCC states pledged to curb greenhouse-gas emissions to keep global warming to under two degrees Celsius (3.6 degrees Fahrenheit) over pre-industrial levels.

Taking effect from 2020, the accord will also channel hundreds of billions of dollars in help to poor countries that are most exposed to climate change.

It was seen as the jewel in the crown of Fabius' long career, during which he became France's youngest prime minister at 37.

In his letter to Hollande, Fabius said: "You gave me the honour of approaching me to preside over the Constitutional Council, and I thank you deeply for this.

"In my opinion, there is no incompatibility (between this post) and the presidency of the last phase of COP21.

"However, given the beginnings of an internal controversy on this subject, I find it preferable to tender my resignation as president of the COP."

The Constitutional Council is a nine-member court that assesses the constitutionality of legislation. Members are appointed for nine-year terms by the president and the presidents of the Senate and National Assembly, the two chambers of parliament.

- 'Two jobs' debate -

Environment Minister Segolene Royal -- Hollande's former companion and long considered a rival to Fabius -- brought the debate out into the open last Friday.

Recently handed the additional role of overseeing "international relations on climate" in a cabinet reshuffle, Royal called for "the rules to be clarified" on holding two jobs.

On Monday, the junior minister for research, Thierry Mandon, said it was "inconceivable" that Fabius should preside both the Constitutional Council and COP21 simultaneously.

"The Constitutional Council is supposed to be above the fray, you can't take on jobs that are with the executive," Mandon argued.

Constitutional experts said the picture was unclear.

There was no precedent for determining whether the two jobs are incompatible, as the COP21 post is unpaid and voluntary, they said.

The COP is the supreme decision-making body of the UNFCCC, which convenes at ministerial level once a year.

The presidency is taken up by the country which hosts the gathering. France will hand on the role to Morocco, which will organize COP22 in Marrakesh November 7-18.

After clinching the deal in December, Fabius's remaining tasks as COP21 chief would have been mainly ceremonial, such as initiating the formal signature of the agreement at the United Nations in New York on April 22.

Detailed questions and the day-to-day running of the presidency are left to Laurence Tubiana, 64, France's special envoy on climate, who also played a key role in the Paris talks.