Mr McCartney was left as the lone voice of the party in the Northern Ireland Assembly when his four colleagues deserted him to form their own new party, the Northern Ireland Unionist Party.

Leader Robert McCartney could be relegated to the assembly backbenches

The split followed a row over party tactics in the event of Sinn Fein being allowed into government in Northern Ireland without prior weapons decommissioning by the IRA.

Anti-agreement

The four dissidents - Cedric Wilson, Patrick Roche, Norman Boyd and Roger Hutchison - said Mr McCartney advocated walking out of the Assembly but they were determined to fight for the anti-Good Friday agreement cause.

Mr Wilson said they had formerly tendered their resignation from the party and had contacted assembly speaker Lord Alderdice to seek official recognition within the body as the Northern Ireland Unionist Party.

The four men withdrew their support for Mr McCartney in December, saying he had made remarks which they saw has "impugned their integrity".

The new party released a statement at Stormont on Tuesday which said their immediate objective was to consolidate the opposition to the agreement within the Assembly.

That opposition, they said would be undermined by Mr McCartney's "exit strategy from the assembly".

The new party will oppose the Good Friday Agreement

The four men said: "For elected members to withdraw from the Assembly on the personal whim of a party leader at a time of maximum crisis for the Union would be an act of gross political irresponsibility.

"The result would be to give the Yes camp within unionism a virtually unchallengable ascendancy within the assembly thus effectively neutralising the opposition of anti-agreement parties remaining within the assembly."

They said Mr McCartney's strategy amounted to a recipe for the destruction of the effective opposition within the assembly to the Good Friday Agreement.

'Bringing down the Assembly'

They said: "For our part we will continue to absolutely oppose the formation of an executive that would include Sinn Fein.

"We are prepared to hold the line on this issue even if that means bringing down the Assembly."

Their new party would concentrate on working with other anti-agreement parties "to retrieve the union from an agreement which legitimises terrorism and concedes fundamental demands of Irish nationalism".