Chancellor George Osborne has refused to rule out cuts to the number of frontline police officers in an interview with the BBC's Andrew Marr.
He said the counter-terrorism budget was being increased and more money would be spent on defence.
But asked to rule out police cuts he said: “Every public service has to make sure it is spending money well.”
Labour said he should heed security warnings from senior officers and do an “11th hour” U-turn on cuts.
Mr Osborne said he had now reached a deal with all departments ahead of Wednesday's Spending Review.
The chancellor denied reports he was still at war with Home Secretary Theresa May over cuts to the policing budget amid warnings from officers that further cuts would threaten Britain's ability to respond to a Paris-style attack.
“In the summer Budget we took the decision to increase our defence spending and increase our counter-terrorism budget,” the chancellor said.
“These were decisions taken before the terrible events in Paris and, of course, those events in Paris throw a spotlight on the threat we face but, of course, that threat existed before that terrible event,” he told the BBC.
“So when it comes to the Spending Review, we are going to make the argument that protecting the British people is our first duty as a government, that economic security is a vital part of national security.
“And precisely because we are making difficult decisions in other parts of our budget, we can give our military more kit, we can increase our counter-terrorism budget by 30%.”
Some of the extra money for counter-terrorism would go to the police “to make sure we can deal with marauding gun attacks,” he said. But he refused to rule out cuts to the number of frontline officers.
“We will make sure Britain is properly defended from the terrorist threat,” he added.
Shadow chancellor's views on policing issue
Labour's shadow chancellor John McDonnell said he supported more money for the intelligence services but was “really worried” about further cuts to police numbers.
He told the BBC: “If he (Mr Osborne) cuts the police service in the way he's planned, that undermines our security.
“I am saying to him directly, end the cuts to policing because all the police chiefs are saying you are putting our communities at risk. I will support him if he says, straightforwardly, policing cuts will not take place.”
Shadow Home Secretary Andy Burnham also urged Mr Osborne to heed security warnings from senior police officers, saying: “He can't be allowed to dig in and damage national security with his dogmatic approach to deficit reduction.”
Will Osborne miss deficit target?
Chancellor George Osborne hinted that his target of running a £10bn surplus by 2020 could be revised down as he deals with worse than expected borrowing figures.
“The precise level of the surplus will be set out in the forecasts on Wednesday,” he told Andrew Marr.
The estimated £20bn in spending cuts to be unveiled on Wednesday are aimed at helping the UK eliminate its deficit and run a surplus by the end of 2020 but economists warned the latest figures on the public finances would make that difficult without more belt-tightening than already planned.
John McDonnell said: “George Osborne's economic and fiscal plans are in utter chaos just months since he announced them.
“We all know he gave up a long time ago on his 2010 promise to eradicate the deficit by today, but now he can't even stick to the deficit reduction plans he announced as recently as July.”