Mr William Hague, the Conservative leader, said his party would support the legislation, but pressed for a review of its effectiveness before it came up for renewal in 12 months' time and suggested internment should be put back on the UK statute book to match Ireland's powers.
Mr Hague called for a promise that no terrorist prisoner would be released early until the process of arms decommissioning had started.
Turning to the Omagh bombing, he told the Prime Minister: "Like you, we utterly reject the excuses of those who carried out this atrocity. Like you, we extend our deepest sympathy to the devastated families involved. Like you, we wish to see no effort spared to bring the evil murderers responsible for this to justice."
Mr Hague said the declaration by the Sinn Fein leader, Mr Gerry Adams, that "the violence we have seen must be for all of us now a thing of the past, over, done with and gone," should be welcomed. But he insisted the "tough words must be matched by clear deeds".
The Good Friday agreement effectively linked progress on decommissioning with the early release of convicted terrorists and membership of the Northern Ireland Executive.
Mr Hague stressed: "If Sinn Fein's words are to ring true, we must now look for the actual dismantling of the apparatus of terror which paramilitaries on both sides have held over the people of Northern Ireland for so long."
He asked Mr Blair: "Can you assure us that terrorist prisoners will not be released early until the process of giving up guns and bombs has actually begun? Don't you agree that it was a mistake last autumn, as we argued then, to remove from our statute books the reserve power of internment - a power that is retained in the Irish Republic?
"Will you agree to a full and published review of the effectiveness of this legislation to be published before coming back to the House to seek its renewal in 12 months' time?"
Mr Blair thanked the Opposition for its support, stressing that the Belfast Agreement had to be read as a whole and not just the bits on decommissioning or prisoner release.
"Under the agreement - contrary to what some people sometimes say, which is that decommissioning is no part of the agreement - it is. There is an obligation in the agreement for all weapons to be decommissioned within a two-year period, and I hope the process begins as soon as possible."