Two Green members of the European Parliament have asked parliament president Antonio Tajani to postpone the vote about a new European Defence Fund until after the EU elections.

They warned against the compromise text of the fund's regulation, which needs approval by the parliament's plenary to become law.

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The controversy has pitted MEPs from the left-leaning side of the house against those from the right.

Specifically, the concern is about the parliament sidelining itself, by handing away scrutiny powers regarding how the proposed €13bn will be spent.

"In our opinion, this act is so serious that it deserves a very strong answer from the European Parliament in order to protect its interests," wrote the Belgian and German Green MEPs Philippe Lamberts and Reinhard Buetikofer, in a letter seen by EUobserver, dated 9 April.

Buetikofer had represented his group in negotiations the EU parliament has held with the European Commission and the Council of the EU (representing national governments).

The three EU institutions reached an agreement about the rules for the new EU defence fund last February, in so-called trilogue negotiations, held behind closed doors.

But just days after the deal was agreed, centre-left French MEP Edouard Martin sent a letter to Tajani, in which he raised "some serious concerns" about the outcome.

The problem was about the way the annual work programme of the EU's military fund would be decided.

The council had pressed for those programmes to be decided through so-called implementing acts, which is a type of legislation that does not require approval by MEPs.

An implementing act is proposed by the European Commission and then approved or vetoed by committees that consist of representatives of national governments.

Whether secondary legislation should be an implementing act or a delegated act - the alternative, through which MEPs do have a say - is a common bone of contention in trilogue talks.

According to Martin, parliament's lead negotiator Zdzislaw Krasnodebski "decided to give in and accepted the adoption of work programmes by means of implementing acts".

"This decision, if upheld, will not only weaken the parliament's scrutiny powers regarding the implementation of a sensitive, 13-billion-euros programme; but it will also create a damaging precedent," said the French MEP.

Krasnodebski is a Polish member who sits with the right-wing European Conservatives and Reformists group.

Less than two weeks after Martin wrote his letter to Tajani, Krasnodebski followed up with a letter of his own.

It was co-signed by Dominique Riquet and Francoise Grossetete, the MEPs who followed the files for respectively the liberal group and the centre-right European People's Party.

The three wrote they were "very surprised" by Martin's letter, and the "unusual nature of this approach".

They defended the decision to agree to implementing acts instead of delegated acts, because there was a "specific and sensitive nature" related to defence policies, and that the member states had "specific prerogatives of sovereignty".

The three MEPs stressed that it would be "extremely damaging" to the EU's interests "if this significant political breakthrough should be postponed to the next mandate, or even permanently compromised".

The set-up of the European Defence Fund (EDF) shows a new direction for the EU, which previously rarely dealt with military matters.

The two Green MEPs followed up the second letter saying that now they were "very surprised" about that second letter's content.

For them, that MEPs will not have a say in how the fund's money is spent is a "severe strategic mistake".

"Precisely because the specific nature of defence and the EDF, that is, the development of lethal technology with the help of EU taxpayers money, there should be on the contrary stronger mechanisms for transparency and parliamentary control," the Green MEPs wrote.

As of Friday (12 April), the plenary debate on the EDF regulation was still scheduled for Wednesday (17 April), with a vote the next day.

It is the last plenary session of the EU parliament before elections in May.

The European Defence Fund is planned to cover the period 2021-2027.

The size of the fund - €13bn proposed by the commission - still needs approval by the member states and parliament in a separate process.