Britain's failure to draw up a Brexit blueprint may mean the country will be forced to remain in the EU's customs union past its own 2020 deadline, lawmakers said Thursday.
Extending the post-withdrawal transition period is the "only viable option", the Brexit parliamentary committee said in a report.
It also criticised Prime Minister Theresa May's efforts to find an alternative arrangement with the customs bloc "despite this being absolutely integral to the future EU-UK relationship."
"We are rapidly running out of time to get new trade and customs arrangements in place," said committee chair Hilary Benn.
May's own cabinet is in deadlock over two options for customs arrangements after the March 29, 2019 withdrawal date -- intended to be installed in the transition period running to late 2020.
The "maximum facilitation" model proposes using trusted-trader arrangements and technology to avoid border checks whilst the "customs partnership" system would see Britain collect tariffs on behalf of the EU for goods heading to the continent.
"Given that ministers are indicating that neither of the two options being discussed are likely to be ready by December 2020, when the transition period ends, the UK will in all likelihood have to remain in a customs union with the EU until alternative arrangements can be put in place," Benn added.
The government has been dismissive of suggestions that the transition period may need to be extended as far as 2023.
"We have been clear that there will be one implementation period that will end in December 2020," a spokeswoman for May said.
"We are obviously working on various positions, including our future customs relationship, and our intention is that we will be ready."
The EU has made it clear that it expects clarity on proposals for border arrangements between Ireland and the British province of Northern Ireland -- one of the largest issues governing the customs crisis -- at an upcoming summit of June 28-29.