Ambulance drivers and staff will go out on a 12 hour strike on Thursday across London and the south east as part of an ongoing national pay dispute.

The strike is due to begin at midday and end just before midnight.

It comes at a time when ambulance services are under massive pressure. Chief executive Ann Radmore resigned last week and was temporarily replaced by the trust's medical director Fionna Moore.

From Jan 21: London Ambulance Service apologises after Croydon woman dies while waiting more than an hour for an ambulance

In a letter to ambulance staff London Ambulance Service branch chairman Eric Roberts said: "This is not a dispute against the LAS, but against the unfair, divisive pay policy of the Government. We have to have our voices heard. This is about the future."

Union members are being encouraged by their union to tweet using #nhspay and #nhsstrike on the day to put pressure on health secretary Jeremy Hunt.

He has offered staff a 1 percent pay rise but only for staff who have not already been given an incremental increase, against the advice of the NHS pay review body saying all staff should get a rise.

Mr Hunt has called on his Labour opposite number Andy Burnham to condemn the strike which he said risked patient safety and which he said was only supported by four percent of staff.

The strike is the third in the past year with days of action on October 13 and a four hour walkout in November 24, when the military were called into action to help ferry patients to hospitals.

From Nov 24: NHS workers out on strike again over 'fair pay' with cost estimated to be £300,000

From Oct 13: Industrial action by ambulance workers to continue all week in Surrey and London

The Evening Standard has reported that a military presence of 10 times more than the November strike is being planned for Thursday with 1,200 soldiers on standby.

LAS spokesman Jason Killens told the paper they were preparing for a worst case scenario saying: "Some ill and injured patients will not get a response, or will have to wait longer for an ambulance."

South East Coast Ambulance, which covers Surrey, said: "In previous industrial action on this issue the vast majority of our staff, while supporting the strike, continued to provide ‘life and limb’ cover for the most serious calls.

"We will continue to respond to life-threatening emergencies but other less serious calls may experience delays."

Hospitals are also making their own contingency plans.

A spokesman for Croydon Hospital said it was watching the situation very carefully to make sure there was as little disruption as possible while keeping services safe.

He said: "This may mean we have no choice but to cancel some planned appointments or routine operations, but we want to avoid this where we can."