British Prime Minister Theresa May speaks as she takes part in a news conference with Turkey's President Recep Tayyip Erdogan, after their meeting at 10 Downing Street in London, Britain, May 15, 2018. Matt Dunham/Pool via REUTERS

LONDON (Reuters) - A source in Theresa May’s Downing Street office has dismissed a media report saying that Britain would tell Brussels it was prepared to stay in the European Union’s customs union beyond 2021.

May’s ministers are deadlocked over a future deal with the bloc and the Telegraph newspaper said Britain could stay aligned with the customs union if technology needed to operate borders was not ready in time for 2021, when a transition period after Brexit ends.

“We agreed in December and in March to a backstop but the proposal put forward by the EU is completely unacceptable,” the person familiar with the situation said.

“It would mean a border down the Irish sea and we could never agree to that. Negotiations are taking place on what a workable backstop might be.

“The PM and the government are absolutely clear once the implementation period is over in December 2020 we will be able to not only negotiate and sign trade deals with the rest of the world but also implement them.”