Former Foreign Secretary Boris Johnson says Britain must not agree to a permanent customs union with the European Union, amid speculation that the government is about to propose such an arrangement to win opposition support for its Brexit deal.

Prime Minister Theresa May is preparing for further talks with the opposition Labour Party as she tries to hammer out a compromise that would avert a damaging no-deal exit from the EU on Friday.

Writing Monday in the Daily Telegraph, Johnson says the customs union proposed by Labour leader Jeremy Corbyn would “enslave” the U.K.

Johnson says in a tweet: “We should not agree to be non-voting members of the EU, under the surrender proposed by Jeremy Corbyn – it cannot, must not and will not happen.”