The government's defence review was a "profound missed opportunity", according to Labour leader Ed Miliband.
He said it failed to offer a "strategic blueprint for our future defence needs" - a claim echoed by former armed forces chiefs Lord West and Lord Boyce.
But other experts say that while the cuts are "challenging", they are not "an insurmountable threat to core capability".
Some 42,000 jobs will go, along with Harrier jump jets and HMS Ark Royal.
Prime Minister David Cameron announced the results of the strategic defence review on Tuesday.
He revealed that:
- overall defence spending will fall by 8% over four years
- the RAF and Navy will lose 5,000 jobs each, the Army 7,000 and the Ministry of Defence 25,000 civilian staff
- axing the Harrier and Ark Royal will mean that no planes will be able to fly from British aircraft carriers until 2019
- planned Nimrod spy planes will be cancelled
- no cuts will be made to support for troops in Afghanistan
- two aircraft carriers will be built, but one will not enter service
Mr Cameron denied the review was simply a "cost-saving exercise", saying it was instead a "step change in the way we protect this country's security interests"
But Mr Miliband told MPs it was a "complete shambles".
"It is a spending review dressed up as a defence review, it has been chaotically conducted, it has been hastily prepared and it is simply not credible as a strategic blueprint for our future defence needs," he said.
In the House of Lords, those criticisms were repeated by Lord Boyce, who was chief of the defence staff from 2001 to 2003.
"I cannot say I welcome the statement on this cash-driven defence review and I certainly can't possibly dignify it with the word 'strategic'," he said.
"It will be viewed with dismay by our hardworking and operationally oppressed sailors, marines, soldiers and airmen."
Lord West, former First Sea Lord, said he was unhappy at the gap in air capacity to be caused by the axing of the Harriers and HMS Ark Royal.
"One wouldn't dream within a hospital where you had old-type scanners for people to say we are going to get new ones in 10 years and we are going to stop doing it for 10 years," he said.
But Conservative leader Lord Strathclyde defended the decisions taken, telling Labour peers: "If there had been a little more strategic economic thinking from the last government we wouldn't be in the state we're now in."
He insisted naval forces would be fully equipped for the tasks they were required to do, but acknowledged there would be "a carrier gap in terms of air cover", adding: "We take no pleasure in taking that decision."
Michael Formosa, from Jane's Strategic Advisory Services, said the cuts could have been much worse.
"The UK will retain ample forces to deploy as part of a coalition operation with UN, Nato or EU allies, " he said.
He added that while the 7,000 cut in Army personnel would not affect current operations in Afghanistan, "this reduction might call into question the ability of the Army to undertake 'another Afghanistan'".
Responding to the review, Geoff Morrell, spokesman for US Defence Secretary Robert Gates, said he was "confident that the UK will have the capacity to provide top-tier fighting forces in Afghanistan and other future missions in defence of our shared interests and security".