The government has been defeated in the Lords over giving parliament a bigger say in the approval of post-Brexit trade agreements.
Peers backed by 308 votes to 261, majority 47, a cross-party amendment to the Trade Bill calling for greater accountability and transparency of deals.
The legislation will enable the UK to forge new commercial ties with other countries after breaking from the EU.
But members of the Lords said negotiating objectives should be put before parliament and approved by both Houses before talks on potential trade agreements start.
The amendment also urged the government to consult the devolved administrations on negotiating objectives and assess the possible impact on human health, animal welfare and the environment.
For the Liberal Democrats, Lord Purvis of Tweed said the change to the Bill during its report stage would help produce a trade policy “fit for purpose” in the 21st century.
The Bishop of St Albans, the Rt Rev Alan Smith, said the UK lagged behind the US, the EU and other countries on transparency and accountability of trade deals.
He hailed the changes to the Bill as “fair and reasonable measures that will protect the interests of local industries across the UK and allow us to strike deals that benefit the entire economy”.
Former lord speaker Baroness Hayman said the amendment would provide “a robust framework for the appropriate scrutiny of international trade agreements”.
For Labour, Lord Stevenson of Balmacara said the UK was the only major democracy which did not allow parliament a role in trade policy and insisted it was unacceptable for this to be “off limits”.
Business, Energy and Industrial Strategy minister Lord Grimstone of Boscobel said the government was committed to a transparent trade policy and comprehensive engagement with Parliament.
He denied it was the government’s intention to “shy away from scrutiny” and insisted “enhanced procedures” had been put in place.
He warned that giving parliament a veto over negotiating objectives would limit ministers’ ability to negotiate in the best interests of UK.
“I do believe that our approach to transparency and openness to scrutiny by parliament and other stakeholders is at least as strong as any other Westminster-style democracy, such as Canada, Australia and New Zealand,” Lord Grimstone added.
Following the vote, Lord Purvis said: “Trade agreements today go much further than just tariffs or quotas, affecting issues such as our food and environmental standards or the ability for our NHS to acquire vital medicines and services.
“These new trade deals will be far too important for the government to agree without proper scrutiny or accountability – it is simply unacceptable.
“The vote in the House of Lords has been clear – the government must negotiate any new trade deal in an open and transparent way, giving parliamentarians the right to scrutinise these agreements.
“The Liberal Democrats will always oppose power grabs by ministers, including any attempts to overturn these crucial changes secured by the Liberal Democrats today.”