London (AFP) –  Campaigners called on Qatar Thursday to change its policy towards migrant labourers preparing for the 2022 World Cup, following an investigation by British newspaper the Guardian alleging "modern day slavery".

The report said dozens of Nepalese workers have died while working in Qatar in recent weeks, raising concerns about the Gulf state's preparations to host the World Cup.

Quoting documents obtained by the Nepalese embassy in the Qatari capital Doha, the Guardian said thousands of Nepalese -- the largest group of labourers in Qatar -- faced exploitation and abuses amounting to "modern-day slavery".

Aidan McQuade, director of Anti-Slavery International, who has seen the documents presented by the newspaper, told AFP the evidence was "certainly highly indicative of a brutal working environment which is not good for anybody.

"It is indicative of forced labour and it seems to even go beyond that. This is an open secret, and there is not a concerted effort (by the Qatari authorities) to stop it."

The Guardian said it had found evidence of forced labour on a huge World Cup infrastructure project, although work has not yet begun on building the stadiums for the tournament.

It said some Nepalese men had alleged they had not been paid for months and had had salaries held back to prevent them fleeing, while a group of 30 had sought refuge in the Nepalese embassy to escape the conditions of their employment.

Some workers complained that employers had confiscated workers' passports and refused to issue identity cards, while others said they had been denied access to drinking water despite the fierce desert heat.

One Nepalese migrant employed at Lusail City, which will include the 90,000-capacity stadium in which the World Cup final will be played, said: "We'd like to leave but the company won't let us."

The body organising the World Cup, the Qatar 2022 Supreme Committee, told the Guardian it was "deeply concerned" by the allegations.

McQuade said the situation would improve quickly if the Qatari authorities introduced three measures.

"We call on the Qatari authorities to get rid of the 'kafala' system which limits employees to one employer," he said.

"They should also allow freedom of association so that workers can negotiate better conditions collectively, and they should set a minimum rate of pay."

FIFA's decision to award the 2022 World Cup to Qatar was controversial. It appears increasingly likely that it will have to be played in the European winter because of the intense summer heat.