The group behind strike-hit Southern Railway has apologised to passengers and insisted its "doors are open" to trade unions as it warned over profits after nearly a year of crippling industrial action.

Go-Ahead, which runs Southern through its Govia Thameslink Railway (GTR) business, said full-year rail profits could be impacted by up to £15 million amid ongoing uncertainty over union relations.

It laid bare the cost so far of "lengthy and significant" strike action at Southern after half-year underlying earnings slumped by 35% at its rail arm to £26.9 million.

The group's GTR division suffered losses of around £5 million, while the group revealed it had paid out £9.5 million in compensation to GTR passengers in the first half.

Shares tumbled by as much as 16% after Go-Ahead's profit alert as it said full-year earnings would be lower than expected across both its rail and bus businesses.

Go-Ahead chief executive David Brown said: "Our customers have suffered months of disrupted travel, impacting work and family lives, and we are sorry that such inconvenience and hardship has been caused."

But he told the group was holding talks with trade unions and that service had improved in the past few weeks on Southern.

He said: "No party wants this to go on forever. Our doors are open and we are trying to have negotiations and we want to reach a resolution."

Southern has been involved in a bitter dispute with unions over proposals for so-called driver-only operated trains, with conductors holding 29 days of strikes in the past year, while drivers have separately walked out on six days due to the row.

The main train drivers' union Aslef had suspended strike action earlier this month, but is now holding fresh talks with Southern after its members rejected a previously agreed deal.

The deal had also sparked criticism from the Rail, Maritime and Transport (RMT) union, which called it a "shocking betrayal".

In response to calls for Southern to be stripped of the Southern Franchise, Mr Brown said: "Taking us off the franchise would not resolve the problem".

The group also came under fire after fares were raised across Southern in the new year, but Mr Brown said annual increases were out of the group's control and were set across the industry by the Government.

He confirmed the group had never made a profit on GTR and said it had been "challenging from the outset".

Go-Ahead is in discussions with the Department for Transport over potential penalties for breaching its contractual obligations due to the poor performance on Southern.

The group said that depending on the outcome of the talks, its full-year operating profits in the division could fall by up to £15 million or rise by £15 million.

Go-Ahead's group-wide underlying earnings fell 13.2% to £74.1 million for the six months to December 31, while bottom line profits were 11.7% lower at £67 million.

It revealed that GTR passenger journeys fell 3.4% and passenger revenue was 6.4% lower in the half-year.

The rail division also includes the Southeastern and London Midland franchises.

Its bus business delivered a 7.3% increase in half-year earnings to £47.2 million, but the group said passenger numbers fell 0.7% amid "challenging" trading in the North East and major city centre retail developments in Oxford.