Thomas Cook call-centre staff in Peterborough and Falkirk face new Sunday shifts and working later into the evening.
The company has begun a 90-day consultation with about 400 staff at the two sites after announcing proposals to change the terms and conditions of those on the customer help desk and in sales.
Customer-help staff are most affected by the extension of overall working hours. They will be asked to switch from working shifts Monday to Saturday 8am-8pm, to working seven days a week, 8am-10pm.
Thomas Cook declined to give further details. However a spokeswoman said: “We have to ensure we have robust call centres, to suit customers who want to call on a Sunday evening and with not too many people working at the wrong time.
“We recognise it is difficult. But this is part of our continuing commitment to ensure we are best positioned.”
The Peterborough and Falkirk call centres handle Thomas Cook mainstream holiday bookings. The company’s call centre in Preston, which deals with specialist products, is unaffected.
Ian Ailles, Thomas Cook UK mainstream chief executive, said: “The proposed changes are part of our continued commitment to ensure we are positioned to respond to the needs of our customers.”