STRASBOURG (Reuters) - The right-left coalition that has controlled the European Parliament for more than a decade may be on the verge of collapse after the socialist leader declined to support a centre-right candidate as speaker of the chamber.

The centre-right European People’s Party (EPP) and the centre-left socialists have shared the five-year presidency of the EU legislature since 2004 under an informal deal meant to strengthen the Parliament’s powers and streamline legislation.

But with euroscepticism growing and anti-system parties emerging across Europe, the socialist group said it would not renew the alliance when the Parliament elects a new speaker in January, when Martin Schulz, a socialist, formally steps down.

“Dear Weber, the cooperation is over,” socialist leader Gianni Pittella told a news conference in Strasbourg, replying to a letter from EPP head Manfred Weber, who had proposed a renewal of the alliance before the January vote.

Pittella said breaking up the grand coalition would help reduce the appeal of anti-establishment parties that gain votes in part by depicting moderate parties of all political hues as pursuing the same agenda.

Pittella, whose socialists are the second biggest grouping in the Parliament, is himself a candidate for the presidency.

The EPP is the largest of eight blocs in the chamber but holds only 215, or 29 percent, of the 751 seats, meaning it needs support from other parties to get its candidate elected. Its members include parties such as France’s Les Republicains and German Chancellor Angela Merkel’s Christian Democrats.

The deal with the socialists saw the EPP support Schulz in his bid for the presidency in 2014, on the understanding that he would serve for 2-1/2 years then hand over to an EPP candidate in the second half of the legislature’s term.

Weber urged Pittella and other pro-EU parties to join the EPP in electing the new president to show cohesion against eurosceptic parties such as Britain’s UKIP, France’s Front National or Italy’s Five-Star movement.

Weber told a news conference that the right-left alliance had been crucial to passing all major legislation in the Parliament in recent years, and without that cooperation the socialists would become irrelevant.

The head of the Greens in the Parliament, Philippe Lamberts, said it was “hard to believe” the grand coalition will effectively collapse.

The EPP will choose its candidate later on Tuesday. The other main parties have not yet declared who they will support for Schulz’s job.