The strikes causing misery for hundreds of thousands of rail passengers are about whether guards should have a few seconds more to ensure the safe despatch of trains, union leaders claimed.

The Rail, Maritime and Transport (RMT) union said it had set out a framework for renewed talks in a bid to resolve the long-running row over the role of guards.

Union members on South Western Railway (SWR) staged their third day of action on Wednesday as part of 27 strikes throughout the month, which is causing travel chaos.

In a letter to the company, RMT general secretary Mick Cash said: "I am writing to ask for your agreement to reopen talks around an operational framework that would allow for the action called for December to be suspended.

"Obviously, it would be in the interest of all concerned to try and reach a negotiated agreement and I have today written to Sir Brendan Barber (chairman of Acas) asking him to use his good offices to conciliate."

Mr Cash said SWR had raised the issue of "dwell times" - the length of time a train is stationary on platforms.

"It is our view that adding three or four seconds dwell time at each station to ensure the safest method of working and despatch is surely a small price to pay to guarantee the safety and accessibility for all, which is the issue at the heart of the dispute.

"Safety must come first in all instances which, as you know, is our guiding principle.

"As the union has said repeatedly, there is clearly a deal there to be done which would cost your company nothing and which would give the safety and accessibility guarantees at the platform/train interface that we have been seeking.

"I would welcome your immediate confirmation of your agreement to attend these urgent discussions under the auspices of Acas."

A South Western Railway spokesman said: "We're pleased that the RMT wants to come back to the table, but we need the RMT to show they are serious about ending these strikes in a way that works for passengers.

"They need to explain exactly what do they want instead of the written agreement they took away from Acas last Thursday, and offer a new solution that safely delivers over 10 million more passenger journeys on time every year.

"We want a guard on every train with a safety-critical role, but we want to enable guards to spend more time helping people in wheelchairs and with buggies get on and off the train, walking up and down all the carriages and ensuring the safety of passengers at times of need."