Parliament is to be recalled on Wednesday and Thursday of next week - coinciding with the already announced recall of the Dail in Dublin - to rush the new legislation through.
Echoing the Irish prime minister, he described the changes as being of a "draconian and fundamental nature".
The key measure in the Government's package will allow a senior police officer to testify that he believes a defendant is a member of a proscribed organisation - particularly the Real IRA, whose bomb claimed 28 lives in Omagh earlier this month. A suspect's silence will be cited as corroborating evidence against him.
The prime minister's visit to Omagh was a muted affair in a rain-swept town which is only beginning on the road towards recovery. Although many businesses have reopened following the bombing, many remain badly damaged.
Mr Blair appeared emotionally moved after speaking to local people, who included doctors and nurses and civic and business leaders. He visited several premises, including an Oxfam shop where people were killed by the blast.
He said: "There is a clear need, now that it is plain that we are dealing with this small wholly unsupported, wholly unrepresentative group of extremists, to tighten our law in relation to what is needed to secure a conviction for membership of a proscribed terrorist organisation." The British and Irish governments had the determination to take whatever measures they could to bring those responsible to justice.
The two governments clearly hope that the new measures will go further than previous laws and allow police to round up known members of the Real IRA and put them behind bars within weeks or months. Since most of the group's members are thought to live south of the border it could be that Gardai in the Republic may bring most of membership charges.
Opposition parties in both Britain and the Republic have pledged their support for the measures. The recall of Westminster will cause some administrative upheaval, especially since the Commons chamber is presently littered with scaffolding. Workmen have been removing asbestos from the space above the chamber.
The new measures are expected to pass through the Commons next Wednesday and the Lords on Thursday.
Mr Blair said proposals on the admissibility of evidence, some suggested in the last 24 hours by Royal Ulster Constabulary Chief Constable Ronnie Flanagan, were still being discussed with Government law officers and others. Even before Mr Blair gave details of the measure they were condemned by Sinn Fein chief negotiator Martin McGuinness as "a massive over- reaction."
In Dundalk, meanwhile, the discovery of more than 40 dumped cassette- type firebombs was seen as the latest in a series of signs that panic might be spreading within the ranks of the Real IRA.
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