A senior RAF officer has warned that cuts to military aircraft numbers would leave the UK vulnerable to attack.
Air Marshal Timo Anderson, director general of the Military Aviation Authority, addressed MPs in a private talk, and gave a stark assessment on cuts expected to be outlined next week.
The RAF is expected to lose significant numbers of jet fighters in spending cuts of up to 10% over four years.
AM Anderson said it would be harder to combat a 9/11-style attack.
The first part of the review - setting out the strategic context - will be announced on Monday, with the main spending decisions to be set out in a statement to Parliament on Tuesday.
Armchair critics dismissed
AM Anderson gave a strong defence of the RAF to MPs, amid fears in the service that it will bear the brunt of defence cuts.
Speaking ahead of a meeting of the prime minister's National Security Council, he said that aircraft such as Tornados and Typhoons were vital, "despite what amateur theorists might assert from their armchairs", the Daily Telegraph reported.
Prime Minister David Cameron was recently quoted as saying it was not right for the RAF to have planes that were ready for dogfights with the Soviet Union air force.
But AM Anderson told MPs that "without such an air defence capability, the UK would not be able to guarantee security of its sovereign air space", and that the UK would be "unable to respond effectively to a 9/11-style terrorist attack from the air."
He also argued that the RAF offered a "flexible political and military tool", whose use was often less costly in every sense than the large-scale commitment of ground forces.
"The high level of investment in high-end combat aircraft and air defence systems by, for example, Russia and China... indicates that the essential requirement for control of the air has not been lost on nations whose future interests and political orientation may not necessarily be well disposed to the UK," he said, according to the newspaper.
The National Security Council met on Tuesday for about an hour. The prime minister's spokesman said "very good progress" had been made.
The council has focused on defence equipment and procurement in recent meetings, and it was expected to look at the broader picture as the review enters its final stages.
This includes what is needed for counter-terrorism and cyber-security, as well as what kind of conventional military forces the country wants for the future.
BBC defence correspondent Caroline Wyatt says all the indications are that the government is keen for the UK to retain its role on the world stage, as well as fulfilling its current commitments in Afghanistan.
The individual services have been increasingly at odds with one another as the strategic defence and security review approaches, with signs that Army numbers are likely to be spared, and that the Royal Navy will get its two aircraft carriers, though may have to sacrifice some of its surface fleet - while the RAF could see many jets and some bases cut.
AM Anderson became the first director general of the Military Aviation Authority when it was established in April. Its role is to ensure that military airworthiness regulations are met.