Hundreds of rail workers at Govia Thameslink Railway (GTR) are to be balloted for strike action.
Rail union TSSA says members - which include platform staff, ticket office staff, train crew, engineers, control, administrative staff and management - will vote on industrial action in a a dispute over pay, job security and conditions.
GTR operates the Thameslink, Southern, Great Northern, and Gatwick Express franchises in and around London.
It runs services to Cambridge, Bedford, Brighton and the South Coast and is a vital commuter artery for the capital. Lines run from Central London to Sussex including London Victoria Station, Three Bridges ROC, Brighton Station and Gatwick Airport Station.
Workers will be balloted on Thursday 15 September, with the window closing on Thursday 6 October.
The union says it wants "a pay rise which meets the rising cost of living, a no compulsory redundancy agreement for 2022, and no un-agreed changes to terms and conditions".
TSSA General Secretary, Manuel Cortes, said: "This is a clear signal to GTR that our members are not willing to be pushed around in the face of an escalating Tory cost of living crisis and a government which wants to slash jobs on the railways.
"I want to encourage our members to vote yes both to strike action and action short of a strike because we know we are in a fight for the future of our railways.
"We have already seen a strong set of ballot results elsewhere in rail companies our union represents and have strike action in the pipeline later this month.
"Given we have already seen a series of walkouts by our brilliant members Ministers and bosses at GTR would do well to get real and address our concerns now, so that further disruption does not take place.
"If not, there is likely to be crippling action on one of the main train networks in and out of London and beyond in the near future."