A confrontation ensued between HSBC and its clerical union on Tuesday over the bank's decision to extend its five day work-week by a day for managers.
HSBC plans to keep 170 of its branches open on Saturdays and extend opening hours in 80 others.
The changes would affect at least 1500 branch managers. The bank also plans on reducing management sickness absence pay to a maximum of 26 weeks and halve the contracted level of benefit.
Britain's biggest trade union, Unite, has written to HSBC asking for a dialogue on a voluntary union recognition agreement for the bank's middle managers, who are not included in it its current bargaining group.
"We are appalled that HSBC has decided to unilaterally impose changes to the contracts of its staff," a spokesman for Unite said.
HSBC has said that the change in the work-week was to accommodate customers' changing banking habits. Currently 330 of its 1,369 branches are open on Saturday.