Despite stressing government caution over the IRA ceasefire, Mr Major yesterday foresook his armoured Daimler Sovereign to walk back to Downing Street from the Commons after Prime Minister's Questions.
His walk - which officials admitted would not have taken place during a maximum security alert - came as the Northern Ireland Cabinet Committee was expected at its meeting tomorrow to indicate that it was now prepared to make the 'working assumption' that the IRA cessation of violence is permanent.
But although the move will pave the way to talks between officials and Sinn Fein, ministers are expected to make it clear that the process can be reversed if the threat of terrorist violence returns.
Mr Major warned there was 'no certainty' that the process could be carried 'successfully through to a conclusion' and added that he intended to take the opportunities 'with care, with caution and in our own time'.
Mr Major's explicit highlighting of the importance of Semtex was the first indication of detailed thinking by ministers on the coming negotiations on a surrender of arms by republicans.
It was read by diplomats yesterday as implying that Whitehall could make information on the whereabouts of Semtex a more crucial test of the IRA's good faith than the surrender of guns - as well as easier to achieve.
Downing Street insisted last night that the whole question of arms 'in the round' would be a subject for the 'talks about talks' if and when they take place. But in an answer to Sir James Kilfedder, Popular Unionist MP for North Down, the Prime Minister declared: 'The question of armaments and especially Semtex and detonators, perhaps more than guns, are crucial issues that will have to be dealt with as we advance this process.'