London Ambulance Service has 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."