The government's claim that the rise of new global powers and shifts in the centres of economic activity will in no way reduce Britain's influence in the world is wholly unrealistic, according to a withering report by senior MPs and peers.

It says oversight by the National Security Council, which includes the heads of MI5 and MI6, is "not sufficiently broad and strategic", and sharply criticises ministers for withholding information to avoid proper parliamentary scrutiny.

The report, released on Thursday by the joint parliamentary committee on the national security strategy, describes as extraordinary the failure of the council to discuss the implications of the collapse of the eurozone or the possibility of Scottish independence.

The criticism is all the more significant since the committee includes a number of former ministers and the former head of MI5, Lady Manningham-Buller.

The National Security Council does not appear to have carried out a proper assessment of the extent to which Britain can rely on its allies in future conflicts, says the report. The MPs and peers make clear they are worried that the US is turning its back on Europe to concentrate on defending its interests in Asia.

"We call on the government to reflect deeply on the long-term implications of the geographical and functional shifts in US policy that are now taking place. It raises fundamental questions if our pre-eminent defence and security relationship is with an ally who has interests which are increasingly divergent from our own," the report warns.

It adds: "The government needs to decide if the UK will continue to be as involved in US military action as we have been in the past if the US focuses on Asia-Pacific. If the US is moving towards viewing Europe as a producer rather than a consumer of security … it raises more questions as to what we can expect from the US and what the US expects from the UK."

The committee says it is unacceptable to be told by the prime minister's new national security adviser, Sir Kim Darroch, that it cannot see a paper from the National Security Council's risk assessment.

"It is not that we particularly wish to see a cabinet committee paper, but we cannot judge if the priority risks are the right ones without more detailed information about how they were arrived at," the parliamentary committee says.

It continues: "We fully accept that some parts of [the paper], particularly those relating to terrorism and hostile countries, are sensitive and must remain classified." But passages covering "pandemic flu, accidents, flooding, and severe space weather, for example – could probably be published".

The report adds: "The government's national security strategy simultaneously recognises the rise of new global powers, shifts in the centres of economic activity, and reduced resources in the UK, while at the same time asserting 'no reduction in influence'. This is wholly unrealistic in the medium to long term and the UK needs to plan for a changing, and more partnership-dependent, role in the world."

Chair of the committee, Margaret Beckett MP, said: "A good strategy is realistic, is clear on the big questions, and guides choices. This one does not. We need a public debate on the sort of country we want the UK to be in future and whether our ambitions are realistic, given how much we are prepared to spend."