Bus workers in Hampshire and Dorset have announced a 24-hour strike on the same day a rail strike is due to disrupt services in southern England.
Staff at Bluestar will walk out on 18 June, the National Union of Rail, Maritime and Transport (RMT) Workers said.
The bus firm said the action could cause a reduced service.
The strike is due to coincide with the start of a previously-announced five-day walkout on South Western Railway.
Train services have been reduced on previous rail strike days called by the RMT in the long-running dispute over the future of guards.
The RMT said there was an 87% vote for action by bus drivers, cleaners and ticket office staff based at depots mainly covering Southampton and the surrounding area.
Regional organiser Mick Tosh said the firm had offered a 3% pay rise, worth about 31p per hour.
He said the union was calling for a "decent" increase as well as an enhanced rate for hours worked over the 40-hour contract.
Mr Tosh said some drivers were expected to work 57 hours a week on a flat hourly rate.
Bluestar said it believed its above-inflation pay offer was "fair".
Managing Director Andrew Wickham said: "If the action is confirmed, Bluestar will continue to run services on 18 June although these may be reduced compared to normal operations."
The firm, owned by the Go-Ahead Group, operates bus services in Dorset, Hampshire, the Isle of Wight and Wiltshire.