HELSINKI — Nokia workers are bracing for what may be the steepest job cuts in almost two decades as the world’s largest maker of mobile phones prepares to start a partnership with Microsoft.

A reduction in research and development activities is set to be announced by the end of the month, with as many as 6,000 jobs at risk, said Antti Rinne of Pro, Finland’s biggest private-sector office-worker union. That would be equivalent to 38 percent of the Finnish company’s global devices R&D work force. Nokia declined to comment on the numbers.

Chief Executive Officer Stephen Elop said in February that Nokia will adopt Microsoft’s Windows Phone 7 as its main smartphone operating system over the next two years, a move triggering “substantial reductions in employment.” As he phases out Nokia’s homegrown Symbian and MeeGo systems, workers haven’t been told who may hang onto their jobs.

“This doesn’t make for very efficient or creative working conditions,” said Kalle Kiili, an engineer in Tampere, a research site that employs 3,000 workers.

At $4.3 billion, Nokia’s 2010 research budget for devices, which include products from basic handsets to smartphones that can edit documents and show movies, is more than twice Apple Inc.’s entire $1.78 billion R&D budget.

Since Apple shipped its first iPhone in 2007, Nokia’s share of smartphone sales by volume has shrunk 20 percentage points to 30.8 percent in the final quarter of 2010, according to researcher Gartner Inc.

At the end of last year, Nokia employed 16,134 people in R&D for devices and services, a company filing showed.