The UK upper house of parliament, the House of Lords, on 20 March voted by 265 to 194 to insist the UK should not withdraw from the European nuclear agreement, Euratom, until a replacement deal is in place. They also supported a plan which would require the UK to report to Parliament regularly on its future arrangements with Euratom. The government, which does not have a majority in the upper house, was defeated by 50 votes on this issue after cross-benchers joined forces with Labour and Liberal Democratic peers to insist on specific assurances over research and development collaboration and the movement of qualifying nuclear material. However, MPs are expected to try and reverse these changes to the Nuclear Safeguards Bill when it returns to the House of Commons (lower house).

The UK government has said it intends to establish a new domestic nuclear regime as well as to negotiate a nuclear agreement with the European Union (EU) after the UK leaves on 29 March 2019. Energy minister Lord Henley said the UK's exit was a "done deal" since the triggering of the formal two-year proceedings for quitting the EU in March 2017 had also started the process of leaving Euratom. He noted that the government was "on track" to secure agreements with the USA, Japan and other nuclear partners after Brexit and anything that risked disrupting this process would be counter-productive.

However, former Labour cabinet minister Lord John Hutton, who is chair of the Nuclear Industry Association, said remaining in Euratom should remain as a "backup" option in case other arrangements cannot be negotiated in time. Some Conservative MPs urged ministers to seek associate membership of Euratom noting that patients' cancer therapies could be delayed if isotopes are unable to be imported. Lord Henley said ministers had agreed to provide quarterly reports to Parliament on the Euratom negotiations. The   government has now been defeated twice in the House of Lords over its post-Brexit plans for nuclear cooperation.