The European parliament has rejected EU commissioner candidates from Hungary and Romania over alleged conflicts of interest, in a blow to the incoming president, Ursula von der Leyen.

The unprecedented move by MEPs ratchets up pressure on Von der Leyen, who must get her team of 26 EU commissioners approved next month if her new commission is to be ready for its launch day on 1 November.

In a closed-door meeting, MEPs on the European parliament’s legal affairs committee rejected Romanian socialist Rovana Plumb and Hungarian Fidesz party appointee, László Trócsányi. Both were deemed to have a conflict of interest and denied permission to take part in hearings in front of the relevant European parliament committee, the next and essential step in becoming a European commissioner.

The committee approved the remaining candidates, allowing 24 would-be commissioners to be questioned in a series of hearings due to begin on Monday.

Von der Leyen and her team of 26 commissioners – one from each EU member state (excluding Brexit Britain) – can only take office after a vote by the European parliament scheduled for late October.

While the parliament usually rejects one or two individuals for a weak performance in the three-hour hearings, this is the first time it has rejected candidates before those public encounters. The show of strength is the result of a 2016 rule change to tighten scrutiny of potential conflicts of interest.

Responding to the decision, Trócsányi said he had answered all the questions asked of him “in a transparent fashion” and warned of legal action. “In the case of the Hungarian commissioner-designate, a political decision was made lacking any factual basis, and I fully intend to take all necessary steps against it.”

The former diplomat, appointed to Hungary’s dream job of commissioner for enlargement and relations with neighbouring countries, had already been warned he faced “a rough ride” from liberal MEPs over his record in constructing Viktor Orbán’s legal order.

Rovana Plumb said she had answered the committee’s questions ‘with clarity and openness’. Photograph: Virginia Mayo/AP

Plumb, nominated to lead EU transport policy, was reported by the Romanian press to have allegedly taken out loans that she later donated to the ruling party. She was rejected by 15 votes to six with two abstentions.

Plumb told the Romanian media she had answered the legal affairs committee’s questions with “clarity and openness” and had not received any official decision from the parliament. She did not immediately respond to the Guardian’s request for comment.

In theory both candidates could still become EU commissioners if they satisfy the parliament they have no conflict of interest in their new roles. But their suspension could prompt their governments to nominate alternative candidates. Any new contenders would have very little time to prepare for the hearings, where they would face a barrage of questions from MEPs on their political vision and fine technical policy detail.

Von der Leyen, who has said she wants to lead “a geopolitical commission”, was a surprise choice to lead the EU executive after EU leaders failed to agree on someone from a shortlist of politicians who had competed publicly for the job.

The parliament’s assertive move could also raise tensions with governments in Hungary and Romania, who have clashed with Brussels over the rule of law.