Spending on services like police, defence, transport and justice could be cut by a third by 2017/18 under Government spending plans, a report warns.

The plans suggest 1.2 million job losses in the public sector by that date, 300,000 more than predicted by the Government's official forecasters, according to the Green Budget published by the Institute for Fiscal Studies.

The respected economic think-tank said Chancellor George Osborne's failure to hit deficit reduction targets means tax rises or "substantial" additional cuts in welfare benefits are likely after the 2015 general election to avoid "hard to contemplate" cuts in Whitehall budgets.

The fiscal position may force the Chancellor to raid pensioner benefits, the NHS, schools or overseas aid, hitherto protected from cuts, said the report.

The IFS said: "Over the last 30 years, tax rises announced in the year after a general election have averaged £7.5 billion. Considering this trend, and in the context of the current fiscal situation, further tax rises following the next election would not be surprising."

Mr Osborne is due to borrow £64 billion more than he planned by 2015, due to the poor performance of the economy, and borrowing is likely to be higher this year than in 2012, the IFS found.

With the public finances failing to come into balance as quickly as Mr Osborne had hoped, IFS director Paul Johnson questioned whether the Chancellor can continue to shield the NHS, schools and overseas aid from cuts.

The Government said it will continue to protect these three areas from cuts in the spending review for 2015/16, now being negotiated but Mr Johnson said extending protection further would mean spending on other departments - like the Home Office, Defence and Environment - falling by a third by 2017/18.

If the budget for defence equipment was protected, as Prime Minister David Cameron has suggested, that figure would rise to 35%.

Whitehall departments have so far relied heavily on job losses to meet the Chancellor's austerity demands, and if they continued to do so at the same rate, 1.2 million public sector jobs could go by 2017/18, compared with the 900,000 forecast by the Office for Budget Responsibility, said the IFS.