Ahead of planned strike action next week, passengers on the Thameslink railway line are being warned to travel "only if absolutely necessary".

Govia Thameslink Railway (GTR) says there will be disruption to its services including cancellations, late starts, early finishes, and overcrowding.

It is because members of the Rail, Maritime and Transport union (RMT) at Network Rail and 13 train operators are striking for three days next week in disputes over pay, jobs and pensions.

GTR says its employees who are members of the RMT did not vote to strike, but says its services will be majorly affected because of signallers, operators, and staff on connecting lines who are striking.

The planned strike days are Tuesday June 21, 23, and 25 but disruption is expected between June 21 and 26.

On strike days, GTR says only a quarter of services across its Thameslink, Great Northern, Southern, and Gatwick Express fleets are expected to run.

On June 21, 23, and 25, two Thameslink services will run per hour between St Pancras International and Bedford, calling at St Pancras, West Hampstead Thameslink, St Albans City, Harpenden, Luton Airport Parkway, Luton, Leagrave, Harlington, Flitwick and Bedford.

Two trains will run per hour between St Pancras International and Luton, calling at St Pancras, Kentish Town, West Hampstead Thameslink, Cricklewood, Hendon, Mill Hill Broadway, Elstree & Borehamwood, Radlett, St Albans City, Harpenden, Luton Airport Parkway, and Luton.

It will also only be two trains running through per hour through Central London towards Brighton.

On June 22 and 24, a Sunday service will be running as will be the case on Sunday June 26.

Between June 21 and 26, no trains are expected to run before 7.15am at the earliest. On strike days, services will end at 6.45pm. Services will run up to 11pm on June 22 and 24.

Thameslink says there will be no replacement buses operating because of the amount of strike action taking place across the railway network.

Transport for London is also expecting Tube strikes on June 21.

Angie Doll, chief operating officer at GTR said: "We’re sorry to say that people should only travel by rail between 21 and 26 June if absolutely necessary due to strike action by RMT members at Network Rail and other train companies.

"Services will start much later than normal and finish early. All this may lead to very busy trains and possible delays. Passengers should plan ahead and think about travelling at another time."

Network Rail says it is working with RMT to find a solution to resolve the disputes linked to the strike action.

Train times for Thameslink services between June 21 and 26 will be available at www.thameslinkrailway.com from Friday June 17, GTR says.