His remarks yesterday were the most public indication yet that the Prime Minister has made up his mind to commit Britain to spending billions of pounds on a new generation of nuclear weapons to replace the ageing Trident fleet.
Mr Blair also said that the decision would be made "in the current parliament" and dropped a strong hint that he means to do it without a Commons vote, implying that the issue will be settled before he leaves Downing Street.
Rebel Labour MPs retaliated with plans to test the strength of opposition by forcing a vote at a private weekly meeting of Labour MPs on 31 October.
The rebels claim that the only way the Government can avoid defeat will be for the whips to pull in the so-called "payroll vote" - the ministers, parliamentary secretaries and others who would face the sack if they voted against government policy.
Mr Blair was challenged during Prime Minister's Questions by the Labour backbencher Paul Flynn, over revelations in The Independent that the decision to update Trident had in effect been made and that preparatory work at the Atomic Weapons Establishment at Aldermaston had begun.
The Government said Aldermaston's budget had been doubled and scientists had been recruited to ensure that the Trident fleet was kept in working order, but a political decision to replace it had not yet been made.
Mr Flynn asked Mr Blair whether he agreed with sentiments expressed by the former foreign secretary Robin Cook, who died in August, that nuclear weapons were "hopelessly irrelevant" for the task of combating terrorism and acting as an international peacekeeper. He also called for a Commons vote before an official decision was made.
Downing Street had been given advance warning of Mr Flynn's question, so Mr Blair had a written statement prepared. But before he began, he made an unscripted comment.
He said: "I'm sure there will be a debate and I have no doubt at all that there will be a great deal of discussion as the months and years unfold. Although I do not think anyone pretends that the independent nuclear deterrent is a defence against terrorism, nonetheless I do believe it is an important part of our defence."
Mr Blair then read out the prepared line: "No decisions on replacing Trident have yet been taken but these are likely to be necessary in the current parliament. It is too early to rule in or rule out any particular option.
"As we set out in our manifesto, we are committed to retaining the UK's independent nuclear deterrent. We will take our decision, ultimately, in the best interests of the country."
Mr Flynn said afterwards: "I thought I asked a very reasonable question. It doesn't seem unreasonable that if you're going to spend between £10bn and £15bn, you should have a vote on it. Since he is not going to allow that, we will have to go about it in some other way to show the extent of opposition.
"I cannot think of any conceivable use that nuclear weapons could have, apart from the prestige they give us. They also undermine our position in international talks. How dare we tell Iran not to develop nuclear weapons, when we are going ahead with updating ours?"