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Rail passengers are set to be hit by a further series of strikes every Saturday in January, a union has said.

Northern members of the Rail, Maritime and Transport (RMT) union have already agreed to strike on every Saturday in December; now the dispute will run on.

The action is part of a long-running dispute about guards on trains.

Northern has called for an inquiry by the Advisory, Conciliation and Arbitration Service (ACAS) but the RMT dismissed it as a "PR stunt".

The union, whose members are to hold their 41st day of strike action on Saturday in the two-year dispute, instead called for "meaningful" talks.

RMT conductors, conductor instructors and train driver members on Northern have now been instructed to take action on Saturday 5, 12, 19 and 26 January.

Northern said strike days would see the company running about 700 trains each Saturday but few services would run after 17:00 GMT.

The company said travellers should check the for the latest information on services.

Correction December 20 2018: An earlier version of this story suggested the RMT had refused to attend talks at ACAS.