The administration of British Prime Minister Theresa May and the European Union have taken what could prove to be a big step on what has been long and arduous road towards seeing Britain formally exit the bloc.

Sources cited by Bloomberg said that negotiators in Brussels agreed to a draft text for Brexit after working through the night on Tuesday, while The Financial Times reported that 10 Downing Street has confirmed a “draft agreement” has been reached.

But the news marks only the beginning of the next step in the process, with May set to face staunch opposition to the deal from hardliners in the UK government.

Tory Brexiters and the Democratic Unionist party were among those to immediately threaten to vote down the deal before having even viewed the more than 400 pages, according to the FT.

One provision which will likely prove controversial is a compromise on the issue of how to avoid a hard border with Northern Ireland.

Erstwhile foreign secretary Boris Johnson, who resigned from May’s cabinet in protest over her handling of Brexit negotiations, called terms that would require Britain to follow EU environment and employment rules “vassal state stuff.”

Before the announcement, Johnson suggested on Tuesday via Twitter that the deal as being negotiated would give the UK “colony status.”

The pound rose versus the dollar on the Brexit news, while UK stocks reversed losses.

European Commission president Jean Claude Juncker previewed the breakthrough earlier on Tuesday, saying that, while Brexit is “a tragedy, a historical error,” the EU will “make the best of it.”