BELFAST (Reuters) - Northern Ireland’s Democratic Unionist Party (DUP) said on Thursday it could not support Prime Minister Boris Johnson’s Brexit agreement with the European Union, complicating his task of getting it approved by parliament.

The party’s 10 lawmakers have propped up Johnson’s Conservative Party-led minority government since a national election in 2017, meaning he would need more support from opposition deputies in a vote on Saturday to get the deal ratified.

After days of talks with Johnson, the DUP - whose name reflects their loyalty to the union of Northern Ireland and Britain - said they could not support the deal because it was not in the British province’s interests.

Under the new agreement, Northern Ireland would remain in the UK customs area but tariffs would apply on goods crossing from mainland Britain to Northern Ireland if they were deemed to be headed further, to Ireland and the bloc’s single market.

The DUP said the customs arrangement was not acceptable within the internal borders of the United Kingdom and there was a danger that over time Northern Ireland would start to diverge on value-added tax as well.

The arrangement “drives a coach and horses through the professed sanctity of the Belfast Agreement,” it said, referring to the 1998 Good Friday Agreement that ended 30 years of sectarian conflict in the province.

The conundrum for Brexit negotiators has been how to prevent the border between Northern Ireland and EU-member Ireland becoming a backdoor into the EU’s single market without erecting checkpoints that could undermine two decades of peace.

The DUP said that, while the new deal marked progress in giving Northern Ireland politicians a say on future customs arrangements, it also opposed allowing them to do so via a simple majority in the currently suspended local parliament rather than needing the support of both pro-British and Irish nationalist parties.

Unionist politicians, including smaller parties who are also opposed to the Brexit deal, lost their majority in Northern Ireland for the first time since the partition of Ireland in 1921 at the last local assembly election in 2017.

Sinn Fein, the largest Irish nationalist party in the assembly, welcomed the agreement and the fact that no one community would be given a veto.

The DUP added that Saturday’s vote on the proposals in the British parliament “will only be the start of a long process to get any Withdrawal Agreement Bill through the House of Commons”.

The head of Britain’s main opposition Labour Party, Jeremy Corbyn, said in Brussels he was “unhappy” with the new deal and would vote against it. Lawmakers in his party said they had been told to vote for another referendum on Saturday.