Bank staff and a delegation of trade union officials from around the world has urged Santander bosses in the US to respect workers' right to join a union.
There is currently no recognised union in the US that represents employees at Santander-owned banking chain Sovereign.
The UNI Finance Global Union that led the international delegation to Boston alleges that some workers who have supported organising have been unfairly dismissed.
UNI called on Santander chairman Emilio Botin to ensure management's neutrality in organising efforts.
UNI official Oliver Roethig said: "Workers at Sovereign want a union but we have heard that their bosses are telling them they will lose their jobs if they try to organise.
"Santander has thousands of unionised workers in many other countries so we are dismayed by what we believe is an anti-union attitude in the US."
The UNI delegation called on Santander to re-employ workers it believes were unfairly dismissed and stop interference with workers' right to unite.
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