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A report into the troubled Southern rail franchise should be published immediately by the government, the RMT union has demanded.

It was commissioned last September and written by Chris Gibb, a non-executive director at Network Rail.

The union claims the Department for Transport (DfT) has kept it "under wraps since the back end of last year".

The DfT said the report could not be published in the pre-election period but would be "in due course".

A spokesman said Mr Gibb was appointed by the transport secretary "to lead a project board to improve Southern services", and that it had received his findings.

RMT general secretary Mick Cash said it was "scandalous" the report "remains under lock and key in a vault at the Department for Transport".

"The RMT is demanding its immediate publication as the stench of the Southern rail fiasco hangs like a cloud over this rotten government and reminds passengers that the alternative option of public ownership is now right up the agenda," he said.

"We were told that the report would be published 'after the election'.

"Well, the election has been and gone and the Tories took a hammering along the length of the Southern routes as the electorate sent out the clearest possible message about their failing transport services."

The union has been embroiled in a long-running dispute with Southern's parent company Govia Thameslink Railway over proposed changes to the role of guards on driver-only operated trains.

In a statement, the DfT spokesman added that it was investing £300m "to improve performance and resilience" on Southern.

"Performance has been consistently better since the new year and making sure passengers keep seeing a reliable and efficient service is a priority for the government and the operator," he said.