"Tonight will be historic but for the wrong reasons. We will oppose the toxic backstop & vote against the WA. It’s time for a sensible deal which governs our exit from the EU & supports all parts of the UK", Foster said on Twitter.
The House of Commons is set to vote on the withdrawal agreement and political declaration on the future relationship later on Tuesday, but a number of Conservative lawmakers have voiced concerns, similar to those of the DUP, and Labour is generally expected to oppose the agreement.
This could lead to checks on goods coming to Northern Ireland from the rest of the United Kingdom.
If the deal fails to make it through Commons, Prime Minister Theresa May will have to submit an alternative solution to the parliament within three days. The amendment setting this deadline was voted through last week by 308 votes against 297, with several Conservative lawmakers slamming Speaker John Berkow for allowing the vote to go on.