Hundreds of Southern trains were cancelled today at the start of a five-day strike in an escalating row over the role of conductors.

Members of the Rail, Maritime and Transport (RMT) union "solidly" supported the walkout, saying they were receiving backing from the public as they mounted picket lines outside stations.

Southern's owner, Govia Thameslink Railway (GTR), apologised to passengers, describing the strike as "completely unjustified", as nine out of 10 trains on an emergency timetable ran on time.

Stations were quieter than expected, but passengers complained of packed trains and a sense of hopelessness at being caught in the middle of the bitter dispute.

The Prime Minister's official spokeswoman said Theresa May "strongly condemns" the strike, but Labour claimed it was becoming increasingly clear that the Government was a barrier to resolving the dispute.

RMT general secretary Mick Cash said: "This action has been forced on us by the arrogance and inaction of Govia Thameslink and the Government, who have made it clear that they have no interest in resolving this dispute or in tackling the daily chaos on Southern.

"Our fight is with the company and the Government who have dragged this franchise into total meltdown.

"We share the anger and frustration of passengers and we cannot sit back while jobs and safety are compromised on these dangerously overcrowded trains."

Trains have been disrupted for weeks because of industrial action and a shortage of staff, which the company has blamed on high levels of sickness.

Govia said it had offered new assurances on jobs to the union, although it is pressing ahead with plans to change the role of conductors later this month.

Chief executive Charles Horton said the strike was "completely unacceptable, unjustified and unnecessary".

In a message to passengers, he said: "We want to run new and modern trains to provide more space and capacity and we want to make essential changes to how we operate, including giving our drivers responsibility for closing train doors so that on-board staff can focus on helping you during your journey."

Bruce Williamson, a spokesman for the Railfuture passenger group, said: "There is generally a problem with overcrowding and resources on the railway, so then adding on a strike is a double whammy for customers."

The RMT condemned "lies and smears" over the guards' ballot, pointing out that 393 members were balloted, with 321 voting (81%) and 306 backing strike action (77% of the total members).

The RMT held a number of strikes at ScotRail over driver-only trains but suspended industrial action last week after a new offer.

ScotRail said conductors would be retained on a new fleet of electric trains which are scheduled to start running from next year.

The RMT told Southern last Friday that it would pull back from this week's action if a similar deal was offered.

Paul Plummer, chief executive of the Rail Delivery Group, which represents train operators and Network Rail, said: "Southern's offer to its staff last week shows there are no risks to jobs or pay. Drivers have been closing train doors elsewhere on the railway for the last three decades so we know that it is a safe way of working.

"The rail industry must modernise to deliver the better service today's customers expect and deserve. This dispute is about changes that would mean a better on-board service for passengers and less disruption when problems hit the railway."

Southern's passenger services director Alex Foulds said: "We would like to thank our passengers for the patience and understanding they are showing during this unnecessary strike, and we apologise to them for the disruption they are experiencing today.

"However, the good news is that the timetable we are operating is 60% of our normal service and these are running well.

"The RMT is causing yet more misery for our passengers, and we call on them to let this strike be the last."

Southern said its strike timetable was still running well as the evening peak approached, with no issues at stations and 88.5% of services on time.

But it added that the figure was a manual one compiled by Southern as the company was unable to rely on industry data which was sho