The London Ambulance Service announced plans yesterday to cut 890 jobs – including 560 frontline staff – over the next five years.

The service needs to make savings of £53m although managers said they hoped most of the reductions would come from natural wastage. The reduction represents almost a fifth of the service's 5,000 workers and will fuel the controversy about the threat to frontline services from the £20bn of savings which need to be made by the NHS over the next four years.

Union leaders attacked the move and one campaign group said it would mean "total carnage" for the capital's ambulance service. But the chief executive of the London Ambulance Service (LAS), Peter Bradley, said they had no choice.

"Unfortunately we are not immune to the financial pressures facing the NHS. With nearly 80 per cent of our budget spent on staff costs it would be impossible to make the savings required without removing posts," he added.

The LAS said it expected to cut the number of frontline posts – those responsible for direct patient care – by 560, with a further 330 jobs going in management and support services. Mr Bradley pledged that compulsory redundancies would be avoided wherever possible, adding: "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."

But Unison's regional organiser, Phil Thompson, said patient safety could be jeopardised, adding: "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."

Geoff Martin, of the pressure group London Health Emergency, said: "These cuts will mean total carnage right at the heart of London's ambulance service."