The coalition's austerity drive will get under way in earnest when the Chancellor unveils some of the biggest spending cuts ever attempted in the Western world.

George Osborne is expected to warn of a "hard road" ahead as he details £83 billion of reductions to tackle the deficit, which will see nearly 500,000 public sector jobs culled over the next four years.

Departments are facing budget cuts averaging 25% as part of the long-awaited Comprehensive Spending Review (CSR), with the Ministry of Justice (MoJ) and Department of work and Pensions set to be among the hardest hit.

Unions reacted with fury after it emerged that more than 14,000 jobs could go at the MoJ alone, most of them front-line prison and probation workers. Legal aid and the courts are also expected to be targeted.

At the DWP, there is speculation that 'universal' handouts such as child benefit and winter fuel payments could be limited to save money, on top of a major crackdown on sickness and housing benefit.

Ministers have also forced the BBC to take over funding the World Service, which was previously bankrolled by the Foreign Office. Together with a six-year freeze in the licence fee at £145.50, it means an effective 16% cut in the corporation's budget.

The least affected is thought to be the Department for Education, which has secured an overall 5% reduction. Even the NHS and Department for International Development, which are protected from the curbs, have been ordered to make sizeable efficiency savings to ensure that cash is not wasted.

Labour leader Ed Miliband said he feared Mr Osborne was taking an "irresponsible gamble" with jobs and the economy.

As he left his London home, Mr Miliband said: "What the Government should be doing is putting in place a plan to reduce the deficit but also to protect growth and jobs in our country. People will be very fearful about what is being announced today - fearful for their jobs and fearful for many of the services that they rely on up and down the country."

But Graeme Leach, chief economist at the Institute of Directors, took a different opinion, saying: "There is far too much negativity around the spending review. It is not the end of the world, far from it. In the long term spending cuts will be positive for economic growth."