Conservative Party leader David Cameron, with Ulster Unionist leader Reg Empey (right), receives a standing ovation
The Conservative pact with Ulster Unionists will give NI voters a greater say in national and international politics, David Cameron has said.
The Conservative leader was addressing the UUP annual conference in Belfast where he received a warm welcome.
The two parties have agreed to select joint candidates in future European and Westminster elections.
Mr Cameron overturned past Conservative rhetoric to insist he had a selfish and strategic interest in Northern Ireland.
"As prime minister I will always honour Britain's international obligations," he said.
"I will continue to work closely and constructively with our nearest neighbours in the Republic of Ireland and I will always uphold the democratic wishes of people here in respect of their constitutional future."
Then to applause from his audience he added: "But I will never be neutral when it comes to expressing my support for the Union."
In 1990 the then Conservative Secretary of State for Northern Ireland Peter Brooke helped launch the peace process by insisting the British government had "no selfish strategic interest" in remaining in Northern Ireland if the people opted for Irish unity.
Mr Cameron earlier admitted he still needed to convince the only UUP MP, Lady Sylvia Hermon, of the benefits of closer ties between the two parties.
"I know how much she admired Tony Blair and I understand her views, I need to convince her ... that this new force is in the interests of the people of Northern Ireland," he told the UUP's annual conference in Belfast.
The Tory-Ulster Unionist partnership has been under discussion since July.
The parties said they were not working on a merger but a partnership which would offer a new political opportunity for voters.
Cameron calls for closer ties with the UUP
Its supporters argue it offers Northern Ireland's voters a say in national and international politics and the chance to elect MPs who could serve in a future government.
UUP leader Sir Reg Empey later told the conference that the new alliance offered the chance to forge a dynamic new unionism.
"It is a relationship that is about much more than mere party politics. It is a constitutional statement," he said.
"It declares and demonstrates that Northern Ireland is not a place apart - not an internal colony.
"It is an outward and visible sign of Northern Ireland's rightful place within the United Kingdom."
The UUP leader characterised his opponents in the Democratic Unionist party as having a "little Ulster" mentality that ultimately worked against unionist interests.
"The Ulster Unionist Party wants to offer the electorate of Northern Ireland something more than a continuation of 'us-and-them' politics," he said.
"Something more than a Balkanisation process which will condemn another generation to elections based on sectarian headcounts and pure self-interest."
The Ulster Unionists now have only one MP, having lost their old dominant position in Northern Ireland to Peter Robinson's DUP.
The first joint candidate will be Ulster Unionist MEP Jim Nicholson, who is already a member of the Conservative group in Europe.
The DUP has criticised the new partnership, arguing that a Tory pledge to contest every seat in the UK threatens to split the unionist vote in constituencies like South Belfast and Fermanagh South Tyrone.
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