Sharan Burrow, general secretary of the International Trade Union Confederation, issued the estimate Friday in a plea for Doha to make "urgent reforms" to the rights granted immigrant workers.
Construction workers from Nepal and India account for the majority of the 1.2 million migrant workers in Qatar, Burrow said, noting 400 of the workers died on average every year.
Between 500,000 and 1 million more foreign workers will be needed to complete work on World Cup infrastructure, Burrow added.
"More than 4,000 workers risk losing their lives over the next seven years as construction for World Cup facilities gets under way if no action is taken to give migrant workers' rights. The annual death toll among those working on building sites could rise to 600 a year -- almost a dozen a week -- unless the Doha government makes urgent reforms," Burrow said.
Despite talks with ITUC in the past two years, Burrow said Qatar had taken "no substantive steps" to improve workers' rights.
"The solutions have been put on the table to recognize workers' rights, build effective and efficient dispute mechanisms and end the kafala sponsorship system that enslaves workers," Burrow said.
Qatar does not collect or publish statistics on the number of deaths or injuries incurred by foreign workers, Burrow said, but their home countries do.
Some 44 Nepalese workers died in two months this summer in Qatar, about half from heart attacks or workplace accidents, the British newspaper The Guardian reported. The Indian ambassador to Qatar said 82 Indian workers died in the first five months of this year and 1,460 complained about labor conditions.
Workers said they were sometimes forced to work in 120-degree temperatures without adequate drinking water and that employers sometimes withheld salaries for several months.
A member of the FIFA board has called for an inquiry. A spokesman for the body's executive committee said FIFA was "very concerned" about the reports.