The police's ability to tackle terrorists would be hampered if budget cuts of more than 10% are imposed on forces, the Government has been warned.
The country's top counter-terrorism officer Mark Rowley said he would be "concerned" if cuts about 10% were imposed on the Metropolitan Police, and warned that slashing budgets by 20% would pass a "tipping point".
Metropolitan Police assistant commissioner Mr Rowley admitted he wrote a letter to Theresa May, which was subsequently leaked, warning about the impact of budget cuts on the ability to tackle terrorism.
Chancellor George Osborne has said counter-terrorism funding will rise by 30% in the wake of the atrocity in Paris and insisted he is "absolutely confident" security services would have sufficient resources to keep the population safe if Islamic State launched a gun and bomb attack in the UK.
But on the eve of the spending review, Mr Rowley warned that mainstream police budgets also needed to be spared from deep cuts.
He told the Home Affairs Select Committee he had written to Mrs May following the Paris attacks, and was "very disappointed" that the letter had been leaked.
"I wrote to the Home Office on the back of the meetings of last weekend with an initial take on the impact of Paris and how it might change some of our thinking, some of our assumptions," he said.
"That was a restricted letter which had some operational detail in it. I'm very disappointed it's been leaked publicly."
Although the counter-terrorism police budget was protected, funding for other services including local forces' firearms units and neighbourhood policing also contributed to the effort to tackle the jihadists, Mr Rowley said.
"In terms of the relevance of policing budgets, whilst we have this ring-fenced budget for counter-terrorism, it's not a separate machine, it depends very heavily on local policing resources," he told MPs.
Highlighting the contribution of firearms units, he said that "if we ended up in a situation as awful as we have seen in Paris" the "armed response capability would be critical".
Police forces' armed capabilities are "probably going to need to get stronger" in response to the threat of a Paris-style attack, he suggested.
Local neighbourhood officers also had a role to play, building trust and confidence in communities and helping the "flow of information" about potential suspects.
"I'm sure there are always going to be more efficiencies we can make in policing, but dramatic cuts that undermine those capabilities would be a concern to me," he warned.
In London, the Met could cope with being forced to make a 10% budget saving, but "substantially beyond that we are concerned about the impact that would have".
That 10% figure would vary across the country, but "dramatically above that would be challenging".
Reports have suggested Mr Osborne is considering cuts of around 20%, something that would "certainly" pass the tipping point of what could be absorbed, Mr Rowley said.
Mr Rowley argued that the police's counter-terrorism effort needed to be boosted alongside extra funding for the security services.
"We need something proportionate alongside that," he said.
"We need more to keep pace with this. The workload slowly builds, the threat has built over the last 18 months particularly. We would see it building in the future."
He added: "We are managing it at the moment at a degree of stretch.
"Clearly we have had frank conversations with the Government. If I get into percentage declarations now, I don't think that helps that debate."
Mr Rowley criticised comments by Surrey Police and Crime Commissioner Kevin Hurley, who warned that officers responding to a terror attack outside the capital would be "massacred" due to being outgunned.
He said there should should be a "calm and measured debate" and "people are making some extreme comments which I think are unhelpful".
Mr Rowley's comments came after a senior officer said police leaders have a "responsibility" to highlight the impact of fresh budget cuts on the capacity to respond to a terror attack.
Chair of the National Police Chiefs' Council (NPCC) Sara Thornton defended letters sent by senior officers, including Mr Rowley, to Mrs May last week.
Opening the inaugural NPCC and Association of Police and Crime Commissioners (APCC) joint summit, Ms Thornton said the atrocity in the French capital has "brought the modern threat from terrorism into sharp focus".
The allocation of money to the police service is a matter for politicians, she said, but added: "As police chiefs, I think that we do have a responsibility to point out the operational impact of fewer officers and staff.
"Many of you are aware that letters were sent to the Home Secretary last week by several chiefs - and it is no secret that we have been discussing firearms response, the role of neighbourhood policing in enabling intelligence flows, and the ability of the service to surge in times of crisis.
"We are accused of saying both too much and too little. Some say we wave the shroud, others that we are silent."