Mr Fox said the country was in debt and was now "having our national security cut as a consequence".
"Who is paying for [the government's] incompetence? Our brave armed forces," he said.
Mr Ainsworth told the House of Commons that there was no overall cut in the £35.4bn defence budget, but that they needed to make "tough choices" to reprioritise spending.
"Acute cost pressures remain," he said.
Mr Ainsworth said the pressure on public finances meant they had to match the defence programme to available resources. The cuts announced include:
• Closure of RAF Cottesmore in Rutland - the biggest employer in the county - and immediate loss of one squadron of Harrier jets, with the rest moving to RAF Wittering before being phased out earlier than the previously planned withdrawal date of 2018.
• 2,500 job cuts in armed forces personnel the MoD says "are not critical to current operations". To be done by slowing recruitment and preventing extensions to service, rather than redundancies
• Independent review to look into cutting more civilian defence jobs
• Withdrawal of the Nimrod MR2 in March 2010, one year earlier than expected, and the slowing of the introduction of the Nimrod MRA4 to 2012 - which will hit RAF Kinloss in Moray
• Plan to reduce the RAF Tornado and Harrier force by a further one or two squadrons, with final decisions to be taken in the defence review due next year. RAF Lossiemouth, in Moray, is the RAF's largest Tornado GR4s base
• Temporary reduction in some aspects of army training
• Earlier than planned removal from service of one Royal Navy survey ship and one mine-hunter. Navy's Lynx and Merlin Mk1 helicopters also to be "retired" sooner than planned.
Mr Ainsworth said the changes would ensure that "those who put themselves in harm's way on our behalf remain properly supported and resourced".
Defence Secretary Bob Ainsworth: "This is a difficult balance to strike"
But BBC political correspondent Reeta Chakrabarti said opposition politicians wanted more detail about exactly where the cuts would come and voiced scepticism about much each would save.
Mr Ainsworth said the new three-year £900m spending plans included:
• Twenty two new Chinook helicopters - the first 10 due to be ready by 2012/13. Takes the total fleet from 48 to 70
• An improved "close combat equipment package" with "state of the art" body armour and night vision goggles being made available to 50% more troops
• More Bowman tactical radios for troops and £80m for special forces communications
• One additional C17 transport aircraft
• The number of Reaper drones - unmanned surveillance aircraft - to be doubled as part of a increased funding to improve intelligence and surveillance
• Improvements to the defensive and support arrangements for the RAF Hercules heavy-lift fleet
Mr Ainsworth said the money announced on Tuesday was on top of £280m coming from the Treasury reserve to increase the number of Husky and Jackal vehicles, and kit to tackle roadside bombs - announced by the prime minister on Monday.
This was separate again to money being spent by the Treasury on urgent operational requirements (UORs), the MoD said.
The defence secretary said the core defence budget had come under pressure from rising fuel and utility prices, pay and pension costs, and the growth in the price of equipment programmes - some of which had doubled in cost since original estimates.
As part of the cuts the Harrier jets based at RAF Cottesmore will move to RAF Wittering before being phased out of service altogether - earlier than the planned withdrawal date of 2018.
The base - which opened in 1938 - holds about 3,000 people and has been home to Harriers since 1999. It currently houses the UK's four operational Harrier squadrons.
Mr Ainsworth's announcement came after months of criticism over the number of helicopters in operation in Afghanistan's Helmand province.
By Caroline Wyatt , BBC defence correspondent
The renewed focus on the frontline will be welcomed by those serving in Helmand. However, the first of the new Chinooks won't start coming into service until 2012 at the earliest. That is when British troops could be looking to start a phased withdrawal from Helmand.
Although 22 Chinooks have been ordered, only around a quarter would be likely to go out to Afghanistan, with the rest needed for training in the UK and others in maintenance.
The new C17 transport plane to strengthen the strained air-bridge between the UK and Afghanistan is also much needed, while the counter-IED measures are crucial.
However, the difficulty some will have with the budget re-shuffling is that it prioritises fighting today's war over preparing for tomorrow's. Nor do the cuts appear to result from a carefully-targeted strategy. Cynics might accuse the MoD of again being ready for the last war by the time it's almost over.
Much worse is still to come. The procurement budget is disastrously over-committed, while the squeeze on public spending means more cuts are inevitable. However, those even tougher decisions have been left until after the general election.
The Chinooks will be manufactured in the US by Boeing, but maintenance and repairs will provide "a lot of work" at Vector Aerospace in Gosport, Hampshire, said Defence Minister Quentin Davies.
He was criticised on Tuesday by Defence Select Committee MPs for not giving the full contract to a British firm.
He told them: "We have to buy the best equipment that money can buy for our armed services. We are buying Chinook because Chinook is simply an incomparable aircraft."
Commander Joint Helicopter Command, Rear Admiral Tony Johnstone-Burt, said Chinooks had proved "invaluable" on operations.
With the recession meaning UK public finances now show a deficit of £178bn, some cost-saving measures have been announced and more huge cuts are expected.
After Chancellor Alistair Darling's pre-Budget report last week the Institute of Fiscal Studies predicted that defence, higher education, transport and housing were most likely to be hit, in order to protect schools and hospitals.
Mr Davies told the BBC decisions always had to be made about what the government wanted and what it could afford.
Asked about the possibility of RAF airbase closures, he said: "If we can get by with fewer bases that will be a very good thing to do. We don't have bases for the sake of having bases; they are not an end in themselves."
Director of the Royal United Services Institute Professor Michael Clarke said the decisions were being made in the absence of a defence review, which had not happened for 12 years and was not due until after the general election.
The announcement came as a report by the National Audit Office accused the MoD of driving up projects' overall budgets through short-term cost-cutting.
On Monday, Prime Minister Gordon Brown announced £150m would be spent on tackling improvised explosive devices in Afghanistan - which have killed 122 of the 237 British service personnel to have died there.
The body of the soldier who became the 100th British fatality in Afghanistan this year, Lance Corporal Adam Drane, has been returned to the UK.
Meanwhile, the first of 500 extra British troops to be deployed to Afghanistan have arrived in the country.
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