George Osborne has been given a last-minute warning of the consequences of deep cuts to police budgets as he puts the final touches to Wednesday's Autumn Statement and Spending Review.

The Chancellor has so far failed to rule out reductions in frontline police numbers as part of the review, which sets out how £4 trillion worth of taxpayers' money will be spent over the next five years - and will explain how the Government plans to make £20 billion worth of spending cuts and £12 billion savings on welfare.

The cuts are intended - with a £5 billion crackdown on tax avoidance - to meet Mr Osborne's pledge to eliminate the national deficit without raising personal taxes. But higher-than-expected borrowing in October has led to predictions he may miss his £69.5 billion deficit target for this year and be forced to downgrade the £10 billion surplus forecast for 2020.

Alongside ministries like Local Government, Justice, Energy and Culture, the Home Office is one of the Whitehall departments expected to face reductions of 20%-30% to their budgets, after Prime Minister David Cameron decided to protect the NHS, international aid, defence and schools from cuts.

But England's top counter-terrorism officer Mark Rowley warned MPs that reductions of 20% or more to police budgets would "certainly" pass the "tipping point" beyond which efforts to combat terror would be undermined.

The Metropolitan Police assistant commissioner told the Commons Home Affairs Committee that, while counter-terrorism budgets were protected, they depended "very heavily" on local policing resources like firearms teams and the flow of information from neighbourhood officers.

Mr Osborne has indicated that he is planning a 30% hike in spending on counter-terrorism, which will see the overall budget hit £15.1 billion over the next five years.

Security services will enjoy a boost in resources in the wake of this month's terror attacks in Paris, with an additional 1,900 intelligence staff to be recruited and special forces to get £2 billion of new equipment.

Meanwhile, the Government's commitment to meeting the Nato target to spend 2% of GDP on defence will allow a £12 billion rise in funding for military equipment, bringing the total to £178 billion over the next decade, as set out by Mr Cameron in the National Security Strategy earlier this week.

The NHS in England has been promised an additional £3.8 billion for frontline services in 2016/17, while ministers have pledged to bring in a fairer funding formula for schools and the Government will continue to meet the United Nations target of spending 0.7% of GDP on international aid.

But non-protected departments face cuts which the independent Institute for Fiscal Studies has calculated will leave their budgets one-third lower in real terms as a result of the Government's austerity programme, sparking predictions of redundancies and closures in quangos and government offices.

Apart from sharing out the pain of the spending squeeze, the biggest decision facing Mr Osborne is over how he softens the impact of planned reductions to tax credits, after the House of Lords threw out his proposal to slash £4.4 billion from the payments to millions of families from next April.

Reports suggest that Work and Pensions Secretary Iain Duncan Smith has seen off an attempt to absorb some of the impact by making the Universal Credit less generous, leading to speculation that Mr Osborne may instead target housing benefit or tax reliefs.