Conservative leader Michael Howard has said Tony Blair's failure over asylum led to ricin plotter Kamel Bourgass being able to commit his crimes.
Kamel Bourgass had failed to get asylum
Mr Howard said Bourgass should not have been in the UK and said the case showed "the chaos in our asylum system".
Home Secretary Charles Clarke dismissed the claim, but admitted "border issues" needed tackling, and said measures should include the use of ID cards.
The Lib Dems warned against casting a slur on all asylum seekers.
Bourgass, who had at least four false identities, has been jailed for murdering Detective Constable Stephen Oake and plotting to spread poisons.
Another four men accused of playing a part in the poison plot were found not guilty.
A further four men had pleaded not guilty, but the prosecution offered no evidence against them, so they were also acquitted.
Mr Howard said: "The tragedy of what happened is that Kamel Bourgass, an al-Qaeda operative, should not have been in Britain at all.
"He was one of the quarter of a million failed asylum seekers living in Britain.
"If Mr Blair had delivered the firm but fair immigration controls promised eight years ago, Bourgass wouldn't have been in Britain.
"He wouldn't have been here free to plot a ricin attack."
But the Tories say the government has little idea who is entering or leaving the UK, despite the terrorist threat.
They want 24-hour security at major ports, a new border police and the detention of asylum seekers who arrive with "suspect documents".
The home secretary said asylum seeking should not be confused with terrorism.
But he said there was a need to press ahead with measures to improve security and asylum processing.
Mr Clarke told BBC News: "We have made major advances.
DC Stephen Oake was murdered in January 2003
"Asylum applications are reduced by two-thirds since 2002, removals are three-quarters higher than in 1997 but... there are a whole series of further measures we need to take to address it."
He said ID cards, stronger borders and the type of anti-terrorism laws passed before the election were all needed.
Labour says it would make an Identity Cards Bill, which it dropped shortly before the election, a priority of its first Queen's speech if it wins the election.
Challenging the Conservatives to support it, Labour's Alan Milburn said: "There is no scope for playing politics on this."
Liberal Democrat spokesman Mark Oaten said ID cards would not deter a determined terrorist.
Mr Oaten said it would be "totally wrong" to use "one isolated case" to cast a slur on all asylum seekers and immigrants.
Bourgass told the Old Bailey he had destroyed his documents before entering the UK and claimed asylum in January 2000 using the name Nadir Habra.
The next month he formally applied for asylum but was not interviewed by immigration officials until 15 August 2001.
The Immigration and Nationality Directorate refused the asylum claim, sending him a letter in a brown envelope on 22 August 2001.
Recipes for poisons and explosives were later discovered stored in the same envelope, along with £4,100 in cash.
Bourgass' appeal against the asylum decision was dismissed in October 2001, when he became liable to be arrested and deported.
But when arrested for shoplifting in July 2002 he told police his real name - so avoiding detection.
In the year Bourgass asylum appeal was dismissed, 70,000 applications were made, of which 40,000 were refused. Only 9,000 people were removed from the country.
Gareth Peirce, the solicitor for four men found not guilty in the trial, called on the government to justify some of its claims.
"There was never any ricin, there were no poisons made," she said.
"One has to consider how was it that all of us in this country were allowed to believe that there was ricin. That there was a substantial plot. That it wasn't an individualist, tiny, failed attempt."
Massoud Shadjareh, of the Islamic Human Rights Commission, accused Tony Blair of linking the ricin arrests at the time to international terrorism.
"The real issue is that terrorism has nothing to do with any particular group, asylum seekers or anybody.
"The question is - are we going to let this politics of fear take over our lives?"