By Jon Silverman
Legal affairs analyst
In commending the new law against glorifying terrorism, the prime minister said juries would understand glorification "when they see it ".
Existing legislation was questioned after recent protests in London
Lawyers are split on this prediction. But even those who agree with Mr Blair point out that this is an unusual and, for some, unwelcome basis on which to frame criminal legislation.
James Libson, head of litigation for Mishcon de Reya, said: "When a judge explains a concept - for example, recklessness - to a jury, there is a precise legal definition to go by.
"But glorification is more of a subjective than an objective concept and success in getting a jury to convict will depend very much on the political climate of the time."
The main impact of the new law is likely to be on groups which praise so-called martyrdom operations and those individuals who openly proclaim their sympathy with the perpetrators of terrorist acts.
The consensus amongst lawyers is that the government is probably right in saying that, as the law stands, it is not possible to prosecute someone who brandishes a placard praising the 7/7 bombers.
This would not count as incitement, unlike the carrying of posters which call for the enemies of Islam to be beheaded which could, in theory, be prosecuted under a number of offences.
So, is the kind of case envisaged by the prime minister likely to succeed before a jury?
James Libson thinks it probably will.
"In the current environment, with heightened security fears following 7/7, jurors may well conclude that the new law on glorification has been broken.
"However, a few years further on, when the present events have become history, a different view might be taken if someone was to praise the 7/7 bombers."
Unlike the Public Order Act, which requires the consent of the Attorney-General before a prosecution can be brought, a prosecution for glorification would be brought after consultation between the police and the Crown Prosecution Service.
However, lawyers point out that the same issue of free speech may be a factor in both cases.
It is likely that when the first prosecution is brought under the new law, there will be arguments about proportionality - in other words, whether a restriction on someone's freedom of expression under the Human Rights Act can be justified in order to punish indirect encouragement of terrorism.
Getting the glorification clause through the Commons may be only a stage in a longer legal process.