Opponents of Britain’s departure from the European Union are floating the idea of setting up a new anti-Brexit political party.

James Chapman, a former top aide to Brexit Secretary David Davis, has become an outspoken critic of Britain’s looming departure from the 28-nation bloc.

He is calling for a new centrist political party because both the governing Conservatives and main opposition Labour parties say they will go through with the decision to leave.

Mr. Chapman said on Friday “there is an enormous gap in the center now of British politics” that could be filled by an anti-Brexit force.

He said that two members of Prime Minister Theresa May’s Cabinet have contacted him to express support.

Former Prime Minister Tony Blair has also called for pro-EU politicians from all parties to unite.

Mr. Chapman, a former journalist who was chief of staff to Mr. Davis until June, tweeted this week that “Brexit is a catastrophe” and called on “sensible” lawmakers to reverse it.

He has suggested the new party should be called the Democrats. But many politicians say Britons democratically voted to leave the bloc and it would be wrong to override the decision.

Britain is currently negotiating its divorce from the EU and is due to leave in March 2019.

Progress has been slow on settling the big early issues, including the status of EU nationals living in the U.K. and the size of the bill Britain must pay to settle its commitments to the bloc.

Meanwhile, UK economic growth is faltering amid uncertainty about what the country’s future trading relationship with the EU will be.