The GMB and Unison lost more than 50,000 members between them in 2021, the latest figures show.
The GMB, led by general secretary Gary Smith, revealed in its annual accounts that membership had dropped from 601,907 in 2020 to 571,127 in 2021— a loss of more than 30,000 members.
Unison, now the UK’s largest union, also saw membership numbers fall by more than 22,000 people this year.
Unison’s membership stood at 1,394,892 in 2021, down from 1,417,637 the previous year.
Other major unions, including Unite and the RMT, have yet to file their accounts. Unite said they would do so in due course.
The GMB and Unison declined to comment on their latest membership numbers.
It is not clear why GMB and Unison saw a slump in membership, but the effects of the coronavirus pandemic, retirement and job changes could be a factor.
However, while some unions have been battling with a long-term trend of decline in membership numbers, some smaller unions enjoyed a surge in subscriptions this year.
Nearly 15,000 people joined the Royal College of Nursing (RCN) between 2020 and 2021, while over 10,000 signed up to the National Education Union (NEU) over the same period.
One source said the unions that had focused on industrial matters were the ones being rewarded.
“Unions that focused on industrial issues and standing up for members during the pandemic have gained in membership,” they said.
“Those that didn’t and seemed ineffectual have lost in membership.”
The figures come as industrial action dominates the political agenda this summer, as the UK faces its worst economic crisis in decades.
On Thursday the rail network was brought to a halt after workers at the RMT and Aslef unions walked out. Further strikes are set for the weekend.
The RCN launched its strike ballot week in protest at the latest NHS pay award. The body had asked for a pay rise of five per cent above RPI inflation — which currently stands at 11.8 per cent — but received an offer it said would leave an experienced nurse more than £1,000 worse off in real terms.
Asked about the growth in membership, a RCN spokesperson told HuffPost UK: “Growth in membership is testament to the RCN’s strength in representing the voice of nursing, with patients at the heart of its work.
“It’s also symptomatic of the nursing workforce crisis, a decade of real-terms pay cuts, and patient safety being put at risk.
“In September we will ballot members on industrial strike action, as enough is enough. It’s through the strength of our membership that we are determined to deliver the outcome all in the nursing profession deserve.”
A NEU spokesperson added: “Our increase during this period was in part the Covid effect, with a number of spikes in the number of joiners during periods of lockdown and general uncertainty.
“The importance of being part of a strong, well-organised union was made crystal clear during the pandemic.”
On Wednesday it was revealed that inflation had hit a 40-year high of 10.1 per cent, largely owing to a spike in food prices and household staples.
A new campaign group, Enough is Enough, has sprung up in reaction to the rising cost of living. It is calling for a real pay rise for workers, a reduction in energy bills and a new wealth tax on the top five per cent of earners in the UK.
At a campaign rally in Clapham on Wednesday night, RMT boss Mick Lynch urged the crowd to “fight back against austerity”, declaring: “the working class is back” and “we refuse to be poor anymore”.
On Thursday morning Lynch warned that Britain could be brought to a standstill by a wave of strikes hitting “every sector of the economy”.
“What you are going to get is a wave of solidarity action, generalised strike action, synchronised action,” he told Sky News.
“People are fed up with the way they’ve been treated. The British worker is basically underpaid and gets no dignity or respect in the workplace.”