Amicus, one of the UK's most powerful unions, has threatened to spread industrial unrest across the financial sector.
The first large-scale strike at a British bank for 40 years is planned for this Friday.
HSBC staff will walk out for 24 hours in a dispute over pay and bonuses for clerical staff. Amicus claims 10,000 staff will strike, causing major disruption and branch closures.
But David Flemming, national secretary at Amicus, warned that it would not stop there.
He said that since the financial union Unifi had joined with Amicus they had the power to challenge low pay that he claimed existed across the financial sector - particularly at clerical level.
"We are likely to see a considerable hardening of the union position against below-inflation pay rises," he said.
The HSBC case shows how easy it is for unions to bring parts of the finance sector to a halt.
Although 10,500 employees were entitled to vote, only 3,000 did so, of whom two-thirds voted for strike action. HSBC employs 55,000 people in the UK, so this high-profile strike was instigated by 20% of the eligible workforce. This amounts to 5% of the total UK workforce.
HSBC has denied Amicus figures concerning the number of bank employees who will receive no pay rise or a below-inflation increase this year.
HSBC spokesman, Richard Lindsay, told Personnel Today there was a record £164m available in 2005 for pay rises and bonuses. He said only 1% of staff would not get more cash because they had been classed as under-performers.