By BBC Northern Ireland political editor Stephen Grimason

Ulster Unionist leader David Trimble found himself back on the list of endangered political species this week.

The news on the Patten Report could hardly have been worse for him.

The Royal Ulster Constabulary name and badge would go and Secretary of State Peter Mandelson had no crumbs of comfort to offer the beleaguered First Minister.

Mr Trimble had launched something of a pre-emptive strike before Mr Mandelson's statement to the Commons, making an angry intervention during Prime Minister's questions.

The outburst had been coming for almost a week.

The previous Thursday the Secretary of State had virtually taken over the Ulster Unionist leader's meeting with Mr Blair in Downing Street to tell him the Patten Report would be implemented pretty much in full.

The received wisdom until that point was that there would be something in the Government's response for unionists and friends of Mr Trimble say he was shocked to discover the real politique.

Decommissioning report crucial

Dissident Ulster Unionists have been queuing up to take potshots at their leader and the RUC problem, combined with no hint of a move by the IRA on decommissioning, makes dismal reading in any analysis of the UUP leader's position.

There has been a great deal of focus on the 12 February meeting of the party's ruling Ulster Unionist Council, all 850 of whom will have been appalled by the Patten Report decision.

The reality, however, is that General John de Chastelain's report on decommissioning due before the end of this month will be an altogether more important date.

The story runs like this:

If the general announces decommissioning has begun then Mr Trimble will easily secure support for continuing in government with Sinn Fein.

If decommissioning has not begun its unlikely the UUP leader will allow himself to face the council with any question mark hanging over his intentions.

The most probable scenario is that in the absence of a move on decommissioning by the IRA he would be prepared to resign as First Minister and pull his three executive colleagues out as well, and collapsing the whole process.

Devolution could be suspended

Confronted with these circumstances the Secretary of State could come to the conclusion that suspending devolution and putting the Assembly and the other institutions into shadow mode would be preferable to a complete collapse.

Mr Mandelson could at that point order the suspension and another review, perhaps chaired by himself and the Irish Foreign Minister rather than Senator George Mitchell who must at this moment be trying to find a nuclear shelter in which to hide.

Mr Trimble could then face his party council arguing the UUP held the moral high ground and that the government had stepped in to punish Sinn Fein for the IRA's refusal to disarm.

That of course begs the question about whether the Ulster Unionist Council would allow their leader to stay even in a shadow government with Sinn Fein.

The republican response would be pivotal.

Decommissioning is an aspiration in the Good Friday Agreement, not a guaranteed commitment.

One source close to the Sinn Fein leadership told me if republicans were seen to be blamed for not delivering on something that was never in the deal the results could be disastrous.

"Republican heads would go down. The British Government would have been seen to have caved in to the Unionist veto yet again. It would cause a massive problem," he said.

It seems we are in for another of those peace process bumpy rides.

Knives are out for Trimble

Mr Trimble might have appeared more relaxed the day after the Patten decision than he was in the Commons but he is still deeply troubled.

The knives are out for him and even though few in the party feel there is a credible alternative leader, that's not to say he might not yet be stabbed - even by accident.

And all the while the clock is ticking, not just towards the de Chastelain report and the UUC meeting, but also towards May this year, the only real deadline in the Good Friday Agreement.

Keeping the show on the road at least until then will be Peter Mandelson's first big test.

He spent much of his first hundred days in office here as an interested bystander. Now he is having to get his hands dirty.