About 35 Royal Mail workers in Lancashire have walked out in protest over a worker who claimed she was sacked despite having a doctor's note.
Claire Sumner, who worked in the cash handling and distribution centre in Fulwood, Preston, said she was fired for failing to return to work after slipping a disc in her back.
She said she suffered the injury while lifting a heavy box as part of her duties 18 months ago.
But she claimed Royal Mail managers told her a doctor's note was not a good enough reason to be off work as the company had its own medical assessment team.
About 35 drivers began an unofficial walk out on Monday morning.
'Unlawful' action
The Communication Workers Union said it would be investigating Mrs Sumner's case and talking to local management in a bid to end the dispute.
The Royal Mail described the industrial action was "unlawful".
A spokeswoman said: "There will be no impact on our services in the Preston area and we urge [the strikers] to return to work.
"We never comment on individual disciplinary cases, but we have a very clear disciplinary process which is subject to an appeal.
"We hope the matter will soon be resolved."