A walkout in protest at a 2.75 per cent pay offer by the bank's normally moderate staff union UNiFI could close scores of branches, while supportive action by the TUC-affiliated Banking Insurance and Finance Union (Bifu) threatens the operation of cash dispensers.
Employees will be striking in the face of dismissal threats from management. Recent legislation allows employers to sack strikers even if the stoppage has been called by a lawful ballot.
A management spokesman said Barclays were "pretty optimistic" that most staff would work normally. Traditional areas of union strength such as Wales and north-west England could be hit hardest.
Less than 60 per cent voted for action at UniFi in a turnout of just over 27 per cent. The result at 8,000-strong BIFU was 53 per cent in favour of strikes with a similar proportion of members voting. But Rob MacGregor, assistant seceretary of Bifu, said support for action had been growing since the ballots.
He said the 2.75 per cent offered would mean that an average employee on pounds 13,000 a year would receive just pounds 6.50 a week extra before tax. After four years of basic-rate increases below the level of inflation, both unions are looking for a rise above the 3.3 per cent increase in RPI.