Arlene Foster warned the Prime Minister the DUP will not "call anyone's bluff" over the Northern Irish border issue as the UK enters the last phase of Brexit negotiations.
Threats about the creation of a hard border between Northern Ireland and the Republic has been deadlocking talks with the European Union since the DUP trashed proposals to keep Belfast closely aligned with the bloc after the UK withdraws officially in March 2019.
Speaking to the Today programme, Ms Foster said: "This is too important for us to be playing around with things. This is a union, this is what brought me into politics.
"This is very important we get this right. The red line is blood red, it is very red."
Asked whether the DUP would be willing to vote down Theresa May's proposed plan in the coming months, Ms Foster said: "We don’t want to be in that position but we are not calling anyone’s bluff around us."
The Northern Irish politician insisted her party would not back plans