LONDON (Reuters) - The small Northern Irish party which props up British Prime Minister Theresa May’s government has withdrawn support in some recent votes on a finance bill because its lawmakers are upset with May’s draft Brexit deal.

If the Democratic Unionist Party abstains with its 10 votes, May would still have a working majority of just three votes in the 650-seat parliament: there are 316 active lawmakers elected as Conservatives, and 313 on the opposition benches.

In a vote on the Brexit deal likely to be the most important in Britain for decades, that means every vote counts.

Following is a breakdown of parliament:

320 IS THE MAGIC NUMBER

Although there are 650 seats in the House of Commons, seven lawmakers from the Northern Irish nationalist party Sinn Fein do not attend, and by convention the speaker and his three deputies do not vote.

So there are 639 lawmakers who are considered to be active, meaning any government needs 320 votes for a simple majority.

CONSERVATIVES = 316

May has 314 active Conservative lawmakers and can probably rely on two others who were elected as Conservatives but expelled from the party’s group in parliament.

In the vote on the Brexit deal, May is facing rebellions from within her party on two sides -- from Conservative lawmakers who support Brexit and think she has compromised too much with the EU, and from those who oppose Brexit.

DUP = 10

The DUP have 10 lawmakers. They oppose the draft Brexit deal, though it is unclear how they will all vote.

OPPOSITION = 313

The main opposition Labour party has 255 active lawmakers. Labour says it is confident none of them will support May’s Brexit deal. However, the government hopes to win the support of some opposition lawmakers who either personally back Brexit or were elected from constituencies that voted for it.

Other opposition parties include the Scottish National Party with 35 seats, the anti-Brexit Liberal Democrats with 12, Welsh nationalists Plaid Cymru with 4 and the Green Party with one. Six independents also sit on the opposition benches.