Strikes by Southern Railway train drivers are set to go ahead next week, causing a fresh wave of travel chaos for hundreds of thousands of passengers.
Aslef confirmed there will be three 24-hour walkouts in the bitter dispute over driver-only trains, despite being urged by Transport Secretary Chris Grayling to call off the action.
The minister cited a report by the Office of Rail and Road (ORR) which he maintained confirmed that Southern Railway's plans for driver-controlled trains were safe.
Aslef and the Rail, Maritime and Transport (RMT) union strongly disputed the report.
The Transport Secretary has asked for the rail regulator to draft a national safety framework to improve the way trains are dispatched across the country, saying he expects the unions to be fully involved.
Mr Grayling said: "Aslef should call off its strike. There are no grounds for the strike to go ahead. The independent rail regulator has confirmed after a further review that driver-controlled trains are safe.
"I want our railways to be the safest in the world. I have requested the ORR set out a national framework for further improvements to the way in which trains are dispatched. I want the unions to be fully involved in this.
"The Government is doing everything we can to limit the impact of this strike on passengers and I have written to the unions again today to offer to meet. No-one is losing their job or earning less as a result of the changes."
Aslef general secretary Mick Whelan replied that he was willing to meet the minister to express Aslef's concerns, but added: "You will know that the current trade dispute has been caused by Southern Rail's imposition of Driver Only Operation without any agreement and, as such, your offer to meet does not provide a rationale for my union to call off next week's strike."
Mick Cash, general secretary of the RMT, said: "The ORR are claiming to be an independent safety body when in fact they are a department of the Government made up of Tory appointees and funded by the transport industry.
"Despite their own reports highlighting over 20 serious safety failures, they claim that, with remedial action, Driver Only Operation can be made safe.
"Any other industry that had over 20 serious safety failures would be put under special measures immediately, but with the Government itself pushing Driver Only Operation, perhaps it's unsurprising that the whitewash of a report has appeared on the eve of a strike."
There will be no Southern services during the strikes next Tuesday, Wednesday and Friday. Three further strikes are planned later in the month.