The Conservative leader, Mr William Hague, cautiously welcomed Sinn Fein's entry into the talks. "Let us hope that, after all the bloodshed and disappointments of the past, Northern Ireland can at last look to a future of peace and prosperity," he said.
But Mr David Wilshire, a former Tory member of the Northern Ireland Select Committee at Westminster, described the move as "a further capitulation to terrorism".
Mr Wilshire said the IRA ceasefire would prove to be "just like the last one and not a single weapon or gramme of Semtex is being handed over. Here we have an invitation to talks five weeks after murder and bombing have been suspended. If they cannot get a united Ireland through these discussions, they will go back to bombing again. I think this is a sell-out of unionism".
Earlier, the UUP leader, Mr David Trimble, told BBC Radio that the prospect of meeting the Sinn Fein president, Mr Gerry Adams, was "repulsive". "If he showed genuine repentance for the evil he has done, then maybe it would be a different matter," he said.
Mr Trimble also agreed with a statement made by the former prime minister, Mr John Major, that the thought of sitting down with "some of those people" made his stomach churn.
A Conservative spokesman on Northern Ireland, Mr Ian Taylor, also called on Sinn Fein to make clear its commitment to the talks and to decommissioning.
"There can be no loosening of the consent process," he warned.
Lord Holme, the Liberal Democrat spokesman on Northern Ireland, called on Sinn Fein to "unambiguously" commit itself to the Mitchell principles and he urged unionists to think about "substantive negotiating demands", rather than "skirting around the process of negotiation itself".