Ministers have already earmarked more than 100,000 civil service posts to be cut as the government sets about reducing its administration costs by a third, a Guardian survey has found.

Government sources identified reductions in posts of 15,000 at the Department of Work and Pensions, 8,500 at the Home Office and between 5,000 and 8,000 at the Department for Environment, Food and Rural Affairs.

Very few individuals have yet been targeted to achieve the 33% across-the-board reductions in administration costs announced at the spending review, but nearly all ministries are still losing people from a previous round of redundancies enacted by the last government.

All Whitehall departments are currently working on plans, due to be delivered by the end of the month, which will set out the principles by which they are going to achieve the dramatic reductions in spending announced by the chancellor last month.

Details will include specific programmes that will be cut or merged and will give the first firm clues as to where the 490,000 public sector jobs losses estimated by the independent Office for Budget Responsibility will fall.

Options to avoid mass redundancies are being considered including reducing people's working hours or even taking voluntary pay cuts in order to preserve jobs. Unions have indicated that they would be willing to discuss these options as a last resort.

Information obtained from every ministry suggests that most are still losing people to the last round of voluntary redundancies, all have seen their workforce drop with the recruitment freeze ordered by the government after the election and some have already set out internally the numbers needed to hit their spending reduction targets.

Up to 853 of the 2,134 posts at the Department for Communities and Local Government are expected to go, the Department for Health is "actively considering" launching a new round of voluntary redundancies and the Ministry of Defence has already said it will reduce its workforce by 42,000, 25,000 of whom will be civilian employees.

Some 270 out of the 520 posts at the Department for Culture, Media and Sport will be lost by 2013. The culture secretary, Jeremy Hunt, has already announced a voluntary redundancy scheme for all civil servants in his department and is looking to reduce the headcount of senior staff from 50 to just 15, largely as a result of the Olympic Games implementation team being disbanded after the event.

Overall, the Guardian was told of more than 103,155 posts that will be lost, including in the armed forces. The most recent official figures for employment in the central civil service is 456,060. Nearly all departments acknowledged that they would be reducing posts, saying that they would be looking for voluntary redundancies to avoid making people redundant on a compulsory basis.

A spokesman for the Ministry of Justice confirmed preliminary estimates of up to 15,000 lost posts. "Where possible, staff reduction will be through natural turnover and voluntary redundancy, avoiding compulsory redundancies if possible," he said.

Jonathan Baume, the general secretary of the FDA, which represents senior civil servants, said: "Clearly jobs are not being filled. Senior posts are quietly evaporating. It will be later in the winter before we get a much sharper picture. There are suggestions people may be asked to go part-time or take pay cuts to avoid redundancies. I'd be surprised if we didn't get these recommendations."