The number of days lost to industrial action almost doubled between 2003 and 2004.
There were 904,000 working days lost in 2004, compared with 499,000 in 2003, Office For National Statistics (ONS) data shows.
The average number of days lost per year over the past decade was 560,000. This is relatively few compared to the mass unrest of the past: an average of 12.9 million days were lost in the 1970s and 7.2 million in the 1980s.
Even so, almost 300,000 workers were involved in disputes last year, almost twice as many as in 2003, the ONS statistics showed. The number rocketed after 200,000 public sector workers went on strike last November over plans to axe 100,000 civil service jobs.
Mark Serwotka, general secretary of the Public and Commercial Services union, warned that more protests were possible over plans to increase the pensions age in the public sector from 60 to 65.
Susan Anderson, the CBI's director of HR policy, said any resurgence in militancy by public sector unions was deeply worrying.