The owners of Southern rail have been ordered to invest £13.4m in services after months of poor performance, the Department for Transport has announced.

The government said the fine for the parent company Govia Thameslink would have been higher, but most of the delays had not been Southern's fault, with strikes and unprecedented levels of sick leave being blamed.

Southern operates services between Ashford and Brighton and some services in west Kent.

A series of strikes leading to cancellations and delays over many months has blighted services over many months, frustrating commuters.

The company, whose services are used by 30,000 passengers a day, has been told the money must be spent on improving services.

In a statement, the company said: “The settlement acknowledges that the industrial action taken by the trade unions has been a major contributor to the disruption experienced by passengers in the past year and we welcome the fact that a significant proportion of our claim was recognised.”

“It also concludes that GTR's performance fell short of what was required in terms of cancellations and the provision of capacity.

"As a result GTR has agreed to invest an additional £13.4m on driving up operational performance and further improving services for passengers.”

Charles Horton, GTR’s chief executive said: “We are pleased that this issue has been concluded, and accept and are sorry that our service levels haven’t been good enough for passengers.

"We run the most congested network in the UK where passenger journeys have doubled in the last twelve years.

"This has meant we have been running services for more and more passengers while also allowing stations to be rebuilt, platforms extended, track and signalling replaced and new trains and technology introduced too."

Govia Thameslink (GTR) took over the Southern rail franchise in 2015 and its contract with the Department for Transport (DfT) runs until 2021.