When the EU parliament's biggest group endorsed European Commission chief Ursula von der Leyen's bid for a second term, no one expected public criticism from a senior member of her cabinet.

But the EU's internal market commissioner Thierry Breton took to social media to question just how much support she has.

"However qualified she is, von der Leyen is only backed by a minority in her own camp. The real question now is: 'Can we really entrust (again) Europe's management to the EPP for five more years, or 25 years in a row'?", Breton wrote on X.

"The EPP itself does not seem to believe in its candidate," the former French finance minister added.

The conservative European People's Party (EPP) backed von der Leyen with 400 votes out of a total of 499 delegates voting during a congress in Bucharest.

Breton referred to a "minority" in relation to the 801 delegates with voting rights at the congress, but the EPP said only 737 delegates were entitled to cast votes for a candidate.

Breton belongs to the centrist Renew group, which has often worked with the EPP since von der Leyen took on the role of commission chief in 2019.

The EPP is forecast to remain the biggest group in parliament after elections across the 27-member European Union between June 6 and 9.

The commission refused to be drawn on Breton's remarks.

"It is clear that this comment is a personal remark and is not made in his capacity as a commissioner," commission spokeswoman, Veerle Nuyts, said.

There are guidelines for commissioners about how to behave during the electoral campaign period, she noted.

Officials "will send a reminder to all commissioners about the guidelines... that every college member must apply", Nuyts told reporters in Brussels.

"We ask commissioners to exercise judgement in the application of these rules," she added.