MPs have challenged a decision to shrink the size of Britain's army by thousands of soldiers as a war rages in Europe, demonstrating the need for large land forces.
The Defence Select Committee also criticised cost-saving plans to retire dozens of tanks and other armoured vehicles before replacements are ready.
The MPs urged the Ministry of Defence at the very least to review the timelines for any changes to avoid creating capability gaps that could leave the armed forces vulnerable.
And they accused defence chiefs of appearing "arrogant and unwilling to learn lessons" from Russia's invasion of Ukraine or from last year's disastrous retreat from Afghanistan.
Tobias Ellwood, chair of the committee, called on the next prime minister to commit more funds to defence.
"It is clear that now is not the time for personnel cuts or budget shortfalls in our armed forces," he said. "We cannot afford for our [armed] services to become poorer and weaker. We need to spend more and spend it wisely."
The cross-party committee has published a report into a major review of defence, security and foreign policy and accompanying papers released by the government last year.
It criticised commanders and mandarins for a reluctance to look again at the conclusions of their work - which set out the future size, capabilities and priorities of the Army, Royal Navy and Royal Air Force - in the wake of the subsequent Afghan withdrawal and Russian war.
"No strategy should be set in stone nor subject to constant revision," the committee said.
"However, there is a need for government to be able to respond to major events… rather than downplaying the potential implications of such geopolitical shocks."
A move that came in for particular criticism was the plan to reduce the size of the army by almost 10,000 troops, from a target of 82,000 to 72,500, by 2025 - a reduction that would diminish the force to its smallest in more than 300 years.
The MPs said it was a worry given - on top of security threats - the UK's already over-stretched soldiers are increasingly called upon to help in non-military emergencies such as floods and the response to COVID.
"We are especially concerned about the proposed cuts to personnel numbers and the effective reduction in mass, particularly since that we are seeing defence being used more and more often as an emergency measure to relieve exceptional pressures on public services and perform such tasks that otherwise might be expected to be carried out by others," the committee said in its report.
At the same time, it noted that General Sir Patrick Sanders, the new head of the army, had described the cuts as "perverse" and that Ben Wallace, the defence secretary, appeared to support a review of the decision.
Separately to the evidence considered by the committee, Mr Wallace told Sky News this week that the army would likely grow rather than shrink if the next prime minister commits to a significant uplift in defence spending at a time of growing threats.