France’s interior minister is under attack for employing his teenage daughters as parliamentary assistants during their school holidays as investigations continue into the former presidential frontrunner François Fillon paying family members from public funds.

Bruno Le Roux was summoned to explain himself to the prime minister, Bernard Cazeneuve, on Tuesday after admitting he paid his children €55,000 (£48,000) of public money over seven years to work as his parliamentary assistants.

The financial prosecutor’s office announced it had opened a preliminary inquiry into the affair on Tuesday afternoon.

The scandal broke a month from the first round of the presidential election and weeks after it was revealed that Fillon, the candidate for the rightwing Les Républicains, paid his British-born wife and children up to €900,000 over more than a decade to work for him.

French parliamentarians are allowed to employ relatives, if the jobs are real and work is actually done. Fillon has denied wrongdoing but admitted he “made mistakes”. He has been officially put under investigation over allegations that his wife, Penelope’s, role was fictitious.

Le Roux, a leading member of French president François Hollande’s Socialist party, is accused of giving his daughters, now aged 20 and 23, 24 short-term contracts, known as CDD (Contrat de travail à durée déterminée) between 2009 and 2016 while he sat in the Assemblée Nationale. The details were revealed on TMC’s Quotidien programme on Monday night.

Le Roux rejected comparisons with the Fillon scandal. “Yes, my daughters worked with me, mainly during the summer or other school holidays, but never on a permanent basis,” he told French journalists.

“We’re talking about a summer job with a parliamentarian. When it’s a case of doing filing or a number of parliamentary tasks, I think it’s a good life experience.”

The eldest Le Roux daughter was 15 when she first worked for her father, for 12 days in 2009, it is claimed. The legal work age in France is 16, unless the person is employed by a parent.

However, Le Roux is under pressure to explain how the date of one contract, during the 2013 summer holidays, coincides with the time his daughter was doing work experience at the cosmetic company Yves Rocher in Belgium.

Le Roux, 51, was appointed interior minister last December, after Hollande reshuffled his government following Manuel Valls’s resignation as prime minister to concentrate on his presidential campaign, which ended in a primary vote defeat in January.

On Tuesday morning, Le Roux cancelled an official engagement to inaugurate a national security and intelligence institute, the Institut National des Hautes Edudes de la Sécurité et de la Justice, at L’Ecole militaire in Paris. He was expected to make a statement during the day.

The affair is particularly damaging for France’s socialist government as, at the height of the Fillon scandal in February, Le Roux was among the first to call for greater transparency in French political life.

The opposition Les Républicains party has demanded a judicial inquiry into the affair. As calls for Le Roux’s resignation grew louder, Jean-Christophe Cambadélis, the head of the Socialist party, told RTL radio the problem was not “employing relatives, but establishing whether they had worked”. He insisted there was no suggestion Le Roux had given his daughters “fake jobs”.

“We don’t force people to resign on the basis of a suspicion,” Cambadélis added.

Marine Le Pen, the leader of the far-right Front National, is also under investigation in a “fake jobs” scandal. She has been accused of misusing €300,000 of European parliament funds to finance FN party staff, including a personal bodyguard.