The boss of beleaguered rail firm Govia Thameslink Railway (GTR) is to resign.
GTR, which runs Thameslink, Southern and Great Northern, said chief executive Charles Horton would remain in post for a short time to oversee the development a temporary timetable.
The RMT said: "The rotten franchise he was steering remains in place."
Announcing his resignation, Mr Horton said: "I recognise that passengers have been hugely frustrated at the significant disruption caused by the introduction of new timetables.
"It is the right time to hand leadership of GTR to a new pair of hands."
RMT general secretary Mick Cash said: "Mr Horton may now have gone but the rotten franchise he was steering remains in place and no change at the top will alter that.
"This whole basket-case operation is a failure on every level."