Theresa May's DUP allies have warned the Prime Minister she has "seriously" broken her commitment to them over Northern Ireland's treatment in the Brexit withdrawal agreement. Here are some key aspects of the DUP's role in the Brexit process explained:
The DUP's 10 Westminster MPs provide vital support for Mrs May's minority Conservative government.
The PM failed to secure a majority in the snap general election she called in 2017, leaving her in need of extra support from another party. The DUP struck a 'confidence and supply' agreement with the Tories in return for extra funding for Northern Ireland, among other concessions.
Added to the Tories' 315 MPs, the DUP give Mrs May enough votes to pass legislation in the Commons. Under the terms of the deal, the party is supposed to back the Government on Budget matters and on confidence votes.
The Brexit withdrawal agreement
After two years of negotiations between the UK and Brussels, EU leaders are scheduled to meet on Sunday to consider the withdrawal agreement unveiled by the PM last week.
If the remaining 27 members agree to rubber-stamp the document, it will go to the European Parliament and British Parliament for ratification. Mrs May will need the Commons to pass the legislation in order for Britain to adopt the agreement.
What happened this week in Parliament?
The DUP joined with Labour to cut the Government's majority to just five in a Commons vote on the Budget. The party also abstained on a series of other amendments to the Finance Bill.
The DUP said its actions were intended to send a "political message" to the Prime Minister.
Why did it do that?
The DUP objects to key parts of the withdrawal agreement that it says treat Northern Ireland differently to the rest of the UK. Sammy Wilson said Mrs May's deal with Brussels breached a "fundamental" assurance that Northern Ireland would not be separated constitutionally or economically from the rest of the UK.