Membership of trade unions fell by 275,000 last year to 6.2 million, new figures show.
The 4.2% reduction from 2015 was the biggest annual cut since current records began in 1995, said the Office for National Statistics (ONS).
The proportion of workers in a union was 23.5% last year, 1.2% down on 2015 and almost 9% lower than in 1995.
Around a quarter of women workers were in a union, compared with a fifth of men.
Union membership in private firms fell by 66,000 to 2.6 million and by 209,000 in the public sector to 3.6 million.
Two out of five union members are aged over 50.
The ONS said trade union membership reached a peak of 13 million in 1979, falling sharply through the 1890s and early 1990s before stabilising.
TUC general secretary Frances O'Grady said: "Cuts to the public sector workforce have meant the loss of many good quality jobs, and recent growth in private sector employment has been characterised by a large proportion of insecure jobs.
"This presents challenges to trade unions for recruitment. We're rising to that challenge, not least because we know there's lots that unions can do to help young workers in low-paid and insecure jobs.
"It remains the case that trade union members get higher pay, and better terms of employment."
Unison general secretary Dave Prentis said his union had increased its membership this year and continues to recruit 158,000 members annually, adding: "This is despite over half a million job cuts in the public sector since 2010 and the unreasonable pressure this Government has put on unions."