Centre-right reveals 2014 EU parliament deal

"The EPP group and the S&D group agree that in the 2014-2019 legislative period of the European Parliament they shall support each other in the election of the president of the European Parliament," the letter says.

"They agree that the S&D group will appoint the president of the European Parliament in the first half of the legislative period and the EPP group in the second half," it adds.

The deal was concluded at the beginning of the term by Weber and Martin Schulz, the outgoing socialist president of the assembly. By joining forces, the parliament's two largest groups were ensuring that their candidates would be elected without having to rely on eurosceptic forces.

Guy Verhofstadt, the leader of the liberal group, the fourth largest in the house, also adhered to the deal, in exchange for some top positions in the house.

But Verhofstadt, as well as S&D group leader Gianni Pittella, now say they are not beholden to this "backdoor" agreement. Both have thrown their hats into the race to lead the assembly for the rest of the mandate.

"Those who break with a good tradition and whose signature is no longer worth anything are shattering the stability of the European parliament as an institution," Weber wrote in an email to his group on Monday.

"Those who break our agreement bear the full responsibility, should anti-European forces gain influence."

Last December, the EPP picked Antonio Tajani, a former European commissioner and Berlusconi ally, in a primary process, but the choice failed to enthuse other groups in the parliament.

With just one week to go to the election of the European Parliament's next president, none of the pretenders has managed to emerge as a favourite.

Weber will hold a press conference on the parliament's presidency on Tuesday at 10.30am.