There were 904,000 working days lost in 2004 compared with 499,000 the previous year, according to data from the Office For National Statistics (ONS).
The average number of days lost per year over the past decade was 560,000.
However, the numbers remain relatively small compared to the mass unrest of earlier years.
An average of 12.9 million working days were lost in the 1970s and 7.2 million in the 1980s.
Even so, almost 300,000 workers were involved in labour disputes last year, almost twice as many as in 2003, the ONS statistics showed.
The number rocketed after 200,000 public sectors workers went on strike last November as they objected to plans to axe 100,000 jobs from the civil service.
Mark Serwotka, general secretary of the Public and Commercial Services union, has warned that more mass protests could be on the way over plans to raise the pensions age in the public sector from 60 to 65.
Susan Anderson, the CBI’s director of human resource policy, said any resurgence of militancy among public sector unions was deeply worrying.