The Conservative-run East Sussex council has written to the prime minister David Cameron to complain about cuts to its budget, saying they will “significantly reduce the quality of life” for people in the area.

The letter, also sent to the chancellor, George Osborne, and Greg Clark, the communities secretary, was signed by the leader of the council, Conservative Keith Glazier, plus the leaders of the council’s other party groupings, including Labour, the Liberal Democrats and the UK Independence party.

The council, which has always been Conservative-led, says it recognises the government’s “continued commitment to reduce the national deficit”, but that the expectations on the local authority to reduce spending were unrealistic.

“In line with the rest of local government, East Sussex county council has delivered over £78m worth of savings and efficiencies since 2010,” the letter reads, adding: “Despite this work to reach a balanced and sustainable budget, we have had to consult on a further £70m to £90m cuts programme for 2016/17 to 2018/19.”

The letter continues: “The cuts will significantly reduce the quality of life for many people in East Sussex. Our proposed cuts have to include preventative services which, while offering a short-term saving, will cost the taxpayer more in the medium term.”

The council argues that the allocation of funding did not reflect the “varying needs” of different areas and that East Sussex – which has a population of around 515,000 – is an ageing county, with the number of over-85s due to increase by 7.5% in the next three years.

“This not only shapes the profile of the residents we need to serve but also their ability to support themselves through action or finance,” says the letter.

“Some of the initiatives that the government seems to believe that councils like East Sussex can rely on to make our budgets sustainable are not realistic. There appears to be misunderstanding in relation to the nature of council reserves, the ability to make significant further efficiencies, the likely returns from further sharing of services and the ability to raise income.”

Osborne announced the decision to axe the central government grant to councils over the next four years in the comprehensive spending review in November, prompting the Local Government Association (LGA) chairman and Conservative peer, Gary Porter, to warn of a £4.1bn a year black hole in the budgets of local authorities.

Cameron was accused of hypocrisy in November after he protested about local authority cuts to public services in his Oxfordshire constituency. A leaked letter showed that Cameron had chastised Ian Hudspeth, leader of Oxfordshire county council, for considering cuts to elderly day centres, libraries and museums.

The prime minister said the proposals for possible cuts were disappointing and suggested the Conservative-run council should look at back-office savings and shared services between the emergency services instead.

A recent consultation on plans to cut £40m from East Sussex’s adult social care services prompted 32,000 people to sign a petition against the cuts, and an additional 14,000 people to sign a petition specifically against cuts to a mental health support service.