MPs on the Brexit committee advise Theresa May to keep Britain in the customs union indefinitely.

The prime minister is reportedly considering keeping Britain aligned with EU customs rules until 2023.

This is because the government and civil service need more time to prepare UK borders for life outside the EU.

However, May should agree to Britain staying in the customs union until an alternative customs arrangement is ready to go, the Brexit Committee's latest report says.

LONDON — Theresa May should keep Britain in the customs union for as long as it takes to implement an alternative customs arrangement, a cross-party committee of MPs has urged.

The Brexit select committee — chaired by Labour's Hilary Benn — has published a report in which it calls on the prime minister to extend Britain's membership of the customs indefinitely.

"We are rapidly running out of time to get new trade and customs arrangements in place," Benn said on Thursday.

"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."

The report says it is "highly unsatisfactory" that May's government is yet to reach a position on what customs arrangement it wants to replace the customs union once Britain has completely left the EU.

On Wednesday the chief executive of HMRC, Jon Thompson told MPs that the government's current proposals — "max-fac" and a new customs partnership — would take up to five years to implement and cost businesses up to £20 billion a year. EU officials have rejected both of the proposals put forward by the UK.

The prime minister is considering asking Brussels to let Britain stay in the customs union for two years on top of the proposed 20-month transition period, in order to have more time to solve the Irish border dilemma.

The report expresses concern that the government is still to set out what it wants its future relationship with the EU to be in a number of areas.

"Twenty-three months after the referendum and fourteen months since the triggering of Article 50, we still don’t know what the UK’s future relationship with the EU will be on trade, services, security, defence, consumer safety, data, broadcasting rights and many other things," Benn said.

The report also calls on both the UK Home Office and EU member states to act with greater urgency to protect the rights of EU citizens living in Britain and Brits living on the continent.

"The Windrush scandal has undermined trust in the Home Office’s ability to register EU citizens and process their applications," Benn said.

"But we are just as concerned that EU member states don’t appear to have begun to plan for or communicate how British citizens will be able to regularise their stay. Ministers should seek urgent clarification from other member states on this because British citizens need to know where they stand."