On 21 February 2021, Foster announced the launch of a judicial review of the NIP as she said it had driven "a coach and horses" through the Act of Union and the Northern Ireland Act 1998, which gives legislative effect to the Belfast Agreement. She takes the position that "Fundamental to the Act of Union is unfettered trade throughout the UK," and that the "new regulatory and customs processes required to bring goods into Northern Ireland from the rest of the UK" are inimical to the Act of Union. She is joined by various members of the DUP along with Kate Hoey, Jim Allister and Ben Habib. The threat of the EU to reinstitute a hard border if not for the customs barrier in the Irish Sea is a problem. The group was joined by Nobel Peace Prize winner David Trimble on 24 February, as he wrote a scathing open letter to Boris Johnson prior to the commencement of proceedings. The group have instructed John Larkin QC, the former attorney general of Northern Ireland. Foster was part of discussions involving deputy First Minister Michelle O'Neill, CDL Michael Gove and Vice-President of the European Commission Maros Sefcovic .