European Commission chief Ursula von der Leyen is facing calls from senior officials and lawmakers to justify her choice of candidate to staff a top new EU role, according to a letter seen by AFP on Thursday.

A backlash has been growing in Brussels since von der Leyen appointed German EU lawmaker Markus Pieper for the position of "SME (small and medium enterprises) envoy" in January, with claims two female candidates were passed over despite being more qualified.

The commission president faces suspicion that she named Pieper in exchange for the support of Germany's Christian Democratic Union (CDU) party for her re-election after the European Parliament elections set for June.

In a letter addressed to von der Leyen dated March 27, four EU commissioners including the bloc's top diplomat Josep Borrell pointed to "questions about the transparency and impartiality of the nomination process".

They called for the EU's College of Commissioners "to collectively discuss the answer to these allegations as well as the possible impact on the next steps in the recruitment process of Mr Markus Pieper as SME Envoy".

Such a discussion, they said in the letter sent to all 27 members of the EU executive, should take place "at the earliest opportunity".

The other three signatories are internal market commissioner Thierry Breton, economy commissioner Paolo Gentiloni and jobs commissioner Nicolas Schmit.

"The president has full confidence in the fact that the process was carried out in accordance with all the relevant procedures," commission spokesman Eric Mamer said Thursday.

Pieper did not respond to the claims but said he was "looking forward" to taking on the role, "even if it is with a heavy heart that I leave the parliament".

Pieper's CDU belongs to the biggest political group in the EU parliament, the European People's Party (EPP) which has nominated von der Leyen as its candidate for a second term as commission chief.

Breton belongs to the centrist Renew group, while Borrell, Gentiloni and Schmit belong to the Socialists and Democrats grouping. Schmit is running against von der Leyen.

EU lawmakers had also raised concerns in a letter sent to the commission in late February.

An EU official speaking on condition of anonymity told AFP the two female candidates -- one Czech, the other Swedish -- had reached the final stage of the recruitment process with better results than Pieper.

An amendment urging von der Leyen to scrap Pieper's appointment will be put to a vote in the European Parliament next week, after a debate in which Mamer said the commission chief would "of course" take part.

Anti-corruption group Transparency International EU has called for an inquiry "into potential breaches of the commissioner's Code of Conduct" while campaign group Corporate Europe Observatory voiced concerns about a "flawed -- and politically convenient -- recruitment process".

No date has been set for Pieper to take up the SME Envoy role, with a salary expected to be around 18,400 euros ($20,000) a month.