By Nick Assinder
BBC News Online Political Correspondent
Mr Howard almost dragged a confession from the PM
So the campaign over the European constitution referendum starts here.
Even before the ink is on the document, let alone dry, Michael Howard and Tony Blair used their weekly question time clash in the Commons to draw the battle lines.
For the prime minister, it was the suggestion that Mr Howard was setting out on a course that would fundamentally undermine the UK's position within the EU.
The subtext is now clear - the Tories want voters to reject the constitution so they can move towards withdrawing from the Union.
Mr Howard's tack was to state that, while he wanted Britain to stay in the EU, he was against the Union having a constitution in any shape or form.
Constitutions, he said, are for countries. In other words, give the EU a constitution and it becomes a country.
And he tried, to some effect, to spike the government's guns from day one.
In the event of Britain voting against the constitution, did he intend to go back into negotiations with the EU about it, he asked.
The prime minister ducked, stating he would do what Ireland did when its voters rejected the Nice treaty.
He didn't spell it out in so many words, but that means he would go back to the EU and try to re-negotiate.
The suggestion here is that the government would keep re-framing the question until it got the right answer from the voters.
In any case, it would not mean Britain had to withdraw from the EU. The status quo would continue, said Mr Blair.
Bullseye, sort of, for Mr Howard.
He more or less got from the prime minister the confession that a "no" vote would not inevitably lead to withdrawal from the Union.
The prime minister looked deeply uncomfortable during these exchanges.
He spotted the trap being laid for him - but just a bit too late.
More to come
And he was then clearly delighted that, shortly after this setback, arch Eurosceptic Bill Cash stood up from the Tory benches to keep the spat running.
Now it was Mr Howard's turn to look uncomfortable as the prime minister took delight in pointing out that Mr Cash would love to see Britain out of the EU.
The implication being that Mr Cash is the real face of the Tory party when it comes to Europe.
There will be much, much more of this to come.