Theresa May has warned a terrorist attack in the UK is "highly likely" and would come without warning, one week after 17 people were killed in Paris.
Addressing the House of Commons, the home secretary said security at UK borders had been increased in the wake of the killings and reassured the public that plans were in place to deal with a "Mumbai-style" terrorist attack.
"The House will recall the attacks in Mumbai in 2008 when terrorists armed with assault weapons and explosives took the lives of more than 150 people," the home secretary said.
"Since 2010, and learning the lessons of that attack, we have improved our police firearms capability and the speed of our military response, and we have enhanced protective security where possible through a range of other measures."
She told MPs three serious terrorist plots had been foiled in recent months and claimed the Paris shootings meant security services should be handed more powers under a communications data bill.
The bill, which is known as a so-called "snoopers' charter" and has been criticised by the Liberal Democrats, would extend the range of data communications companies have to store for 12 months.
It would also allow authorities to trawl through social media, online messages and voice calls as well as communications from internet games for the first time.
May said delaying the bill, which was proposed in 2012, was "putting lives at risk".
"Unfortunately, when it comes to communications data and the intercept of communications, there is no cross-party consensus and therefore no Parliamentary majority to pass the legislation to give the police and security services the capabilities they need," she said.
"But let me be absolutely clear. Every day that passes without the proposals in the Communications Data Bill, the capabilities of the people who keep us safe diminishes.
"And as those capabilities diminish, more people find themselves in danger and – yes – crimes will go unpunished and innocent lives put at risk."
Responding, shadow home secretary Yvette Cooper criticised May for revising the bill in 2012 after intelligence and security committee reported on it.