Sinn Féin's four MPs occupy their offices at Westminster today following the lifting of the bar on the party using parliamentary facilities. The abstentionist members are still refusing to take their seats due to objections to the oath of allegiance to Queen Elizabeth.
However, the Ulster Unionist leader says he believes it to be only a matter of time before they take their seats in the House of Commons. He said the party was moving away from its traditional ideals.
Mr David Trimble told the BBC at the weekend: "I have no doubt that the time will come when Sinn Féin representatives, whether it is Mr Adams or Mr McGuinness, I know not, will take the oath and will take their seats in the normal way," he said.
"It will be interesting to see what explanation Sinn Féin will give as to how walking into Westminster is a step towards a united Ireland when it very obviously is a step in the opposite direction."
It is understood the party's president, Mr Gerry Adams, and its Education Minister, Mr Martin McGuinness, will be given offices while newly-elected members Mr Pat Doherty and Ms Michelle Gildernew will share an office.
Mr Trimble renewed his demand for the IRA to continue the decommissioning of illegal weapons. He has said he will take action if there is no further action by the IRA by the a.g.m. of the Ulster Unionist Council in early March.
Speaking from a meeting in Navan, Co Meath, where leading Sinn Féin members were debating the response to the first act of decommissioning by the IRA last autumn, Mr Gerry Kelly said his party was in no mood to discuss Mr Trimble's demand.
He cited the murder of postal worker Mr Danny McColgan and the resurgence in loyalist violence, saying the public was more concerned with these issues than the next step by the IRA.