His initial point of contention is the unnatural alliance between Italy's anti-establishment Five Star Movement and the eurosceptic British UKIP.

In 2014, they set up the Europe of Freedom and Direct Democracy (EFDD), a populist Eurosceptic political group, which Leinen describes as fake.

"They even openly admitted that they have nothing to do together, they have no political affinity, they have no common political programme, they even have no common meetings and they are not voting in the same direction so it is real misuse of parliament's privileges," he told EUobserver.

Being a European political group secures both public financing and lawmaking influence. Groups are allowed to table amendments at the plenary stage and appoint key posts on legislative files.

In 2016, over €58m from the European Parliament budget was distributed to the political groups - of which the EFDD obtained some €3m.

The 2019 budget has increased the overall purse to €64m.

Doru Frantescu director of VoteWatch Europe, a think-tank in Brussels, told this website that the EFDD doesn't function as a normal political group.

"In fact a member of the Five Star and UKIP vote against each other slightly more often than they vote together, so more than half of the time they vote differently," he said.

He says the lack of political cohesion in the EFDD is striking, when compared to all the other groups.

The far-right Europe of Nations and Freedom (ENF), mostly composed of a French National Rally MEPs, comes second after the EFDD in terms of the lack of political cohesion, he says.

In an emailed statement, spokesperson Hermann Kelly said that the EFDD does have a common political charter to which all MEPs and parties in the group subscribe and that regular meetings among members are also held in Strasbourg and Brussels.

"Like other smaller groups in the European Parliament, we are routinely discriminated against by the eurofederalist groups," he said.

'Political affinity' definition

Leinen is seeking to impose a 'political affinity' criteria in the parliament's internal rule book, which includes making sure that at least three-quarters of the group is composed of the same European party.

The whole would be overseen and monitored by either the Bureau, a body responsible for the internal workings of the parliament, the Conference of Presidents where the leaders of the parliament's political groups meet, or the plenary.

Last week, a plenary vote in Strasbourg was postponed on the issue given the objections from the Greens who fear it could backfire later on.

"I am very reluctant to make a political body make a political decision about whether another political group should exist on the basis of subjective criteria," said Max Andersson, a Swedish Green MEP.

He said the rules on political groups need "to be fair for everybody, even for those we do not like."

Leinen concession

Leinen now says he is willing to remove the three-quarters rule as a concession to the Greens in the hopes of securing an agreement and a plenary vote later on this month.

"I think we will replace this idea by having a common political affinity," he said.

He says people wanting to create a political group must show, for example, that they have common objectives and a common programme.

They also need to vote together and meet together, in what he describes as practical issues that "prove you are a group".

This article was updated at 13:12 on Tuesday (22 January) to reflect an emailed statement from the spokesperson of the EFDD group