The world footballing authority conceded on Thursday that there was little it could do to remedy the ordeal for migrant workers in Qatar reduced to slave labour conditions in creating the infrastructure for the 2022 World Cup. But a Fifa board member dismissed all talk of reversing the decision to hold the tournament there even if conditions were not improved.

A European parliament hearing on the scandal, first revealed by the Guardian last year, was told that Qatar was a slave state for migrant workers, scores of whom had died in appalling conditions while or after working on the building sites of the Gulf country.

Theo Zwanzinger, Germany's member of the Fifa executive, admitted conditions for migrant workers in Qatar were "absolutely unacceptable", agreed that human rights considerations should play a bigger role in the decisions of world and European football authorities, and added that Fifa would be carrying out detailed and independent monitoring of the working conditions surrounding the World Cup building boom. But there could be no going back on decisions already taken.

"This feudal system existed [in Qatar] before the World Cup," he said. "What do you expect of a football organisation? Fifa is not the lawmaker in Qatar."

Human rights activists and trade unionists demanded a more forceful role by Fifa at the meeting, which also heard wrenching testimony from the French-Algerian footballer Zahir Belounis, who had his passport confiscated and was denied an exit permit because of a dispute with his Qatari club. He was trapped in Qatar for two years largely unpaid and unable to leave.

"I just wanted to go home," he told the hearing. "I'm the victim of a system of modern slavery."

Much of the criticism of Qatar centred on the so-called kafala system in the labour market, which essentially confers quasi-ownership rights over employees or migrant labourers, a form of neo-feudalism.

The Qatari World Cup organisers submitted a 50-page report this week pledging to improve the plight of migrant workers. Chairing the meeting, the German Greens MEP Barbara Lochbihler said the Qatari promises were welcome but that a more systemic policy was needed rather than piecemeal remedies.

The Qatari government sent a letter to the parliament in Brussels on Thursday stating that about 2,000 companies had been blacklisted last year and almost 500 so far this year for questionable labour market and employment practices.

MEPs complained at the hearing that the Qatari embassy in Brussels had declined to take part in the hearing and that firms recruiting workers for contracts in Qatar had also stayed away, as did Uefa, the European footballing authority.

Zwanzinger signalled that there were frictions within Fifa and Uefa over how to deal with political, ethical and human rights issues.

"If you continue to run the World Cup in a state which enslaves workers, it shames the game. The government must end of the system of kafala if the World Cup is to be played in Qatar in 2022," said Sharan Burrow, general secretary of the international trades union congress. "We want hear from the Fifa president and executive committee about how they will guarantee that the World Cup must only go ahead if there is legal reform to kafala and for workers' rights. The new charter from World Cup organisers in Qatar sets out sham conditions, without even any means to ensure that companies comply."

Zwanzinger declined to give such assurances from Fifa.

"That would be absolutely counter-productive," he said. "Pressure and threats won't achieve much … We have a duty. The decision has been taken to grant the World Cup to Qatar, whether I like it or not."

Burrow said: "Qatar is a slave state for 1.4 million migrant workers. It doesn't have to be that way. Qatar chooses to build its modern nation with the labour of migrant workers and deliberately chooses to maintain a system that treats these workers as less than human."