Long-awaited reform of Qatar's controversial exit visa system, which requires foreign workers to obtain their bosses' permission to leave the country, came into force on Sunday, the government said.

“Law No 13 of 2018... regulating the entry, exit and residency of expatriates is being implemented starting today,” the interior ministry announced on Twitter.

Qatar announced in September it had approved legislation to scrap the visa system — a lynchpin of the country's “kafala”, or sponsorship, system which many liken to modern-day slavery.

Under the new law, all but five per cent of a company's workforce — reportedly those in the most senior positions — can leave without prior permission from employers.

Those not allowed to leave Qatar “for any reason” can file a complaint to the Expatriate Exit Grievance Committee that will “take a decision within three working days”, the ministry said.

Some anxious workers took to government social media websites on Sunday, to ask how they could find out if they were among the five per cent.

Qatar's labour minister, Issa al-Nuaimi, said he was “exceptionally pleased” with the implementation of the new law.

“We are proud that the state of Qatar has become an example and a model for labour reform in the region,” he told AFP.

Nuaimi added that Qatar wanted to give workers in the country, the “best standards possible”.

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Scrapping the exit permit is the biggest announcement made so far since Qatar agreed last November to enter into a three-year agreement with the UN's International Labour Organization (ILO) to oversee reform.

The ILO's Houtan Homayounpour, head of the labour agency's project office in Doha, said on Twitter that the reform “will have a direct and positive impact on the lives of migrant workers”.

The football World Cup 2022 host has come under intense pressure to reform its labour laws, which have been repeatedly denounced by human rights groups.

Critics have long argued for the abolition of the exit visa system.

Research published last year by rights group Migrant-Rights.org found around a quarter of all exit visa requests were denied by the government.

Vani Saraswathi, associate editor and director of projects with Migrant-Rights.org, said the reform was “long overdue and welcome”, but added a warning.

“We have to be cautious about celebrating it as a huge development,” she said.

“Passport confiscation is still rampant and the law also allows for employers to hold passports with the workers' permission.” She also noted that the law does not cover domestic workers, often seen as the most vulnerable.

Rights groups have argued that without the proper safeguards, relaxation of the visa system could lead to unscrupulous employers holding workers' passports.

A study earlier this year by Qatar University's Social and Economic Survey Research found that 53 per cent of migrant workers said their bosses held their passports.

There are some two million foreign workers in Qatar.