The Ulster Democratic Party was expelled yesterday from the cross- party talks on Northern Ireland for its links with the killings of Catholics. Colin Brown and David McKittrick examine the latest threat to the peace process.

Gerry Adams last night warned that the expulsion of the Ulster Democratic Party from the cross-party talks over links to the sectarian murders of Catholics in the Province had not defused the crisis facing the peace process.

The UDP, led by Gary McMichael, walked out of the talks before they were expelled, but they could be back within three weeks. Mo Mowlam, the Secretary of State for Northern Ireland, said they would have to show "by word and deed" that they were committed to the peace process. David Andrews, the Irish foreign minister, said ministers and officials would remain in contact with the UDP.

Both governments were reluctant to expel the UDP, fearing it would destabilise the talks process, but their removal became inevitable when the parties gathered at Lancaster House, in London, and united in calling for ministers to uphold the Mitchell principles requiring a renunciation of violence. "You cannot have people murdered and ignore it," Ms Mowlam said. Mr Andrews said the talks would not collapse, pointing out that Mr McMichael had given his commitment to the peace process.

The sombre assessment of many observers in Belfast is that the UDP's departure from the talks may make the UDA readier to resort to violence. The nightmare scenario is that it could in time lead to a breakdown of the ceasefire declared by the UDA and other loyalist groups in 1994.

It has always been an imperfect ceasefire, with more than a dozen killings by various paramilitary groups and mobs during 1997. But it is generally reckoned that the ceasefire and the presence of loyalist representatives at the talks table have had an inhibiting effect on the killers. The authorities will now be watching anxiously for any evidence of an immediate violent backlash from the UDA against yesterday's development.

The UDA killed three Catholics in recent days before declaring last Friday that its bout of killings had ended.

Tony Blair will attend the talks today in an effort to keep up the momentum for peace, which was threatened with being stalled over the spiral of sectarian killings.