A general view of the Court of Justice of European Union in Luxembourg | EPA/Julien Warnand UK government: European top court could have role during Brexit transition UK ministers had previously said the court’s involvement was a red line.

LONDON — The U.K. could be subject to judgments by the European Court of Justice (ECJ) during a Brexit transition "for a limited time," the prime minister's official spokesperson has confirmed.

The government had in the past been adamant that leaving the EU must also mean escaping the power of the Luxembourg-based court over U.K. affairs, but the shift appears to reflect a realisation that it will be impossible to negotiate a transition period post-Brexit that does not involve the ECJ.

"[Brexit] transition rules could involve the ECJ for a limited time, but obviously that is all a matter for negotiation," the prime minister's official spokesperson told journalists.

He was responding to comments Monday morning from the first secretary of state, Damian Green. When asked on the BBC Radio 4 Today program if the U.K. could remain subject to the ECJ during a transition period, he said: "If there needs to be some kind of implementation period, or transition period, in certain areas after March 2019, which I think everyone agrees is quite likely, then the rules that operate during that transition period will by definition not be the rules that we have afterwards. "

Clarifying Green's remarks, the spokesperson said, "We have said that there could be a period of implementation in a number of areas to avoid any cliff edges. As to what that looks like, that is all a matter of negotiation. Damian Green said 'could', that is the position."

Addressing grass-roots Tories at her party's annual conference in Birmingham last fall, Prime Minister Theresa May set leaving the ECJ as a red line in the Brexit talks.

“Let’s state one thing loud and clear: we are not leaving the European Union only to give up control of immigration all over again. And we are not leaving only to return to the jurisdiction of the European Court of Justice. That’s not going to happen,” she said last October.

But since the general election May has come under fire for her position. At the weekend, for example, three former cabinet ministers called for a more "open" approach and dismissed the idea the U.K. could totally cut ties with the ECJ.

In his BBC interview Monday, Green made it clear that Britain would eventually leave the ECJ, suggesting it was "what people voted for in the referendum."

"If we said now we're going to stay inside the single market which will mean that the European court will decide some basic questions that happen inside this country, I think people would feel that we hadn't left the European Union."