Theresa May's Brexit plans could suffer a parliamentary setback as peers look set to push for the UK to remain in a customs union after leaving the EU.
The House of Lords resumes debate on the Government's flagship Brexit legislation on Wednesday with an alliance of Labour, Liberal Democrat and crossbench peers - along with Tory rebels - eager to soften the Prime Minister's approach.
Labour has warned the Government opposition peers "won't be shy" about inflicting defeats in the House of Lords.
Attempts to rewrite the European Union (Withdrawal Bill) include a cross-party amendment which would require ministers to negotiate "an arrangement which enables the UK to continue participating in a customs union with the EU" after Brexit.
The amendment's sponsors include former Tory party chairman Lord Patten of Barnes and former UK ambassador to the EU Lord Kerr of Kinlochard - the diplomat who drafted Article 50, the legal mechanism for Brexit.
Other battlegrounds in the Lords include measures to limit the Government's ability to use so-called Henry VIII powers, which restrict Parliament's ability to scrutinise future legislative changes required as a result of Brexit.
A cross-party effort to strengthen the "meaningful vote" promised on the Brexit deal will also feature in debates as the Bill goes through its report stage over six days between April 18 and May 8.
Other potential flashpoints include amendments relating to Northern Ireland and a plan to scrap the Government's decision to fix Brexit for 11pm on March 29, 2019.
Shadow Lords leader Baroness Smith of Basildon said: "As we begin the Report stage, Theresa May and her ministers still have an opportunity to bring forward sensible changes in response to concerns raised previously in the Lords.
"A failure to do so however, will amount to kicking the can down what could be a very rocky road. And our peers won't be shy about sending amendments to the Commons, giving MPs a further chance to scrutinise the detail of the Bill."
Shadow Brexit minister Baroness Hayter said the proposed change "forces the government to prioritise a customs union with the EU - not only to boost industry's ability to trade with the UK, but to ensure there is no hard border in Northern Ireland".
Downing Street maintained its opposition to a customs union, which would restrict the UK's ability to strike trade deals after Brexit.
The Prime Minister's official spokesman said Mrs May "has been very clear that the British people voted to leave the EU and expect us to be able to sign trade deals around the world, operating our own independent trade policy".
"As the PM has set out, that means we are leaving the customs union," the spokesman said.
The Government could try to overturn any defeats inflicted in the Lords when the Bill returns to the Commons later this year.