Go-Ahead has warned that its rail division will miss full-year financial forecasts because of continuing strike action on Southern Rail.
It owns 65% of the Southern franchise through Govia Thameslink Railway (GTR) and expects its entire rail division to fall "slightly below" estimates.
For the first six months of the year, turnover on GTR services is forecast to slide by 4%.
Journeys on Southern have been severely disrupted by strikes.
Poor service is expected to continue on Thursday and further industrial action could take place on Friday unless GTR can reached an agreement with the rail drivers union, ASLEF.
Joe Spooner, transport analyst at Jefferies, told the Reuters news agency that Go-Ahead could be facing a further £3m hit from the strike action.
Go-Ahead said: "The GTR team continues to work towards a resolution to these issues and provide the best possible service to customers under the circumstances.
"In doing so, we expect to incur additional costs in this financial year."
The company also said its full-year results would be affected by increased costs from bidding for rail franchises in overseas markets.
Go-Ahead also operates the Southeastern and London Midland rail services, where it expects revenue to rise by 2.5% and 5.5% respectively for the first half of the year.
The company added that its bus business was performing well and first-half revenues were forecast to rise by about 1%.