London Ambulance Service today announced plans to axe 890 jobs over the next five years to make savings of £53 million.
The service said most of the reductions would come from natural wastage and would include 560 frontline positions.
Union leaders attacked the move and one campaign group said it would mean "total carnage" for the capital's ambulance service.
The reduction represents almost a fifth of the service's 5,000 staff and will fuel the controversy over the Government's controversial health reforms.
Chief executive Peter Bradley said: "Unfortunately we are not immune to the financial pressures facing the NHS. This means all areas of our business will face closer scrutiny as we look for ways to make savings while improving the care we give to patients.
"But with nearly 80% of our budget spent on staff costs it would be impossible to make the savings required without removing posts."
LAS said it expected to reduce the number of frontline posts - those responsible for direct patient care - by 560, with a further 330 posts removed from management and support services. Compulsory redundancies will be avoided wherever possible, it said.
Mr Bradley continued: "We are confident that the large majority of posts can be reduced by not filling vacancies. We have an average turnover rate of 7%.
"As part of our planned response we will be introducing a number of measures to control payroll costs, including tighter control of recruitment and reduced use of agency workers.
"We are committed to managing these reductions so that the impact on staff is minimised and at the same time creating an improved and efficient service for patients."
Unison's regional organiser, Phil Thompson, said: "This is being forced on the service by the Government. These cuts are so deep they may not heal. If allowed to be carried out they will put at risk the many Londoners who rely on the LAS every day. With demand escalating and nearly 1,000 fewer staff no one can now be sure of a safe service."
London Ambulance Service worker Eric Roberts, a Unison official, added: "I am shocked by the size of these cuts. This is a cull of highly trained staff. It is disgusting, morally wrong and unacceptable. Have they forgotten the Olympics?
"How will we cope with the biggest sporting event in the world with hundreds fewer skilled staff? All staff are frontline. Without the crucial back-up services we cannot save lives. So much for David Cameron's promises."
Geoff Martin, chairman of London Health Emergency, said: "These cuts will mean total carnage right at the heart of London's ambulance service. There is no question that lives will be put at risk as people dial 999 for an ambulance that just isn't there. This is the Con-Dem health cuts in the raw."
London Ambulance Service receives more than 1.5 million emergency calls every year and responds to over one million incidents.
Last year, it responded to 43,500 incidents more than the year before - an increase of demand of 4.5% - and has a budget of £281 million.
NHS chief executive Sir David Nicholson said: "It is critical that the NHS uses efficiency savings to make real improvements in the quality of care for patients - there is no excuse to reduce services for patients when the NHS will receive an extra £11.5 billion of funding. Every penny saved from measures taken to reduce costs will be reinvested in patient care."
Labour's London Mayoral candidate, Ken Livingstone, said David Cameron had promised before the general election to protect front line services, adding: "London's NHS feels like it is under siege. London's ambulance staff do an amazing job under great pressure often in very difficult circumstances, saving hundreds of lives a year.
"I believe it is crucial that Londoners know there will be an immediate response when they dial 999 and that the capacity of the ambulance service to keep Londoners safe is not being compromised by the cuts and broken promises of this Conservative-led government."