The brutal terrorist killing of a man on the streets of London sparked calls for the Government to revive plans to extend internet and email monitoring powers to the security services.
Former independent reviewer of terror laws Lord Carlile said it should provoke a "pause for thought" on the decision to drop the planned Communcations Data Bill from the Queen's Speech.
And Labour ex-home secretary Lord Reid said such measures were "essential" to combating terrorism, warning it could otherwise take "some huge tragedy" to show the decision was wrong.
Full-scale legislation was dropped from last month's Queen's Speech in the face of Liberal Democrat opposition to what critics dubbed a "snooper's charter".
Prime Minister David Cameron had warned scrapping the plans would put national security at risk by making it harder to bring terror suspects and other criminals to justice.
But Deputy Prime Minister Nick Clegg blocked the change, insisting the wide-ranging proposals would create a "treasure trove of data" that was neither workable nor proportionate.
Ministers are now in talks with internet firms over alternative law changes.
Speaking on BBC2's Newsnight yesterday, Lord Carlile said: "We have to learn proportionate lessons from what has occurred.
"We mustn't rush to judgment. But we must ensure that the police and the security services have for the future the tools they need which will enable them to prevent this kind of attack taking place.
"I hope that this will give the Government pause for thought about their abandonment for example of the Communcations Data Bill and possibly pause for thought about converting control orders into what are now called Tpims, with a diluted set of powers.
"Lone wolves, even though they are always inevitably connected at least with internet training, are very difficult to catch so we must give the authorities proportionate tools to catch them."
He said he was "not suggesting they had anything to do with (yesterday's) attack".
"But I am suggesting that the powers that existed in the past make it more likely that other events can be prevented in the future," he said.
"This may be a small example of something much bigger which could happen. We must ensure the laws are fit for purpose."
Lord Reid said mobile phone data had been crucial in foiling the 2006 plot to blow up airliners using liquid explosives but that terrorists now used online communication.
"Had we not had that method of connecting people through their communications, 2,500 people would probably have been blown out of the sky over the United Kingdom. It was a vital component.
"But now people have moved on from mobile phones to internet, email, text, Skype. We don't have the means of doing what we did six years ago.
"That is where some of the measures the Government has refused to implement, like data communication, is absolutely essential for effective fighting of terrorism. "You will never find out whether you are right on this one until there is some huge tragedy that might have been averted if they had updated the communication appraisals that can be carried out at GCHQ."
Nick Pickles, director of privacy and civil liberties campaign group Big Brother Watch, said: "It is wholly wrong for Lord Carlile and Lord Reid to be arguing for a change of policy before the details of what has happened in Woolwich are clear and before even the family of the victim had been notified. At this time our thoughts should be with the victim's family and not on scoring political headlines.
"Lord Reid was one of those responsible for the knee-jerk decision to try and introduce powers for people to be detained for up to 90 days without trial by the last government, after the 7/7 attack. That should be a clear warning of the dangers of rushing forward policy changes when the nation is in shock and of those who seek to use the politics of fear.
"The current Government made clear in the Queen's speech it will bring forward proposals to address the important issue of identifying who is using a particular internet address and they are right to do so. We face down terrorists by defending our values and traditions and acting proportionately, which is a balance current policy recognises."