The government has been defeated twice in the House of Lords over its plans for nuclear co-operation after Brexit.
Peers 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 backed a plan requiring the UK to report to Parliament regularly on its future arrangements with Euratom.
MPs are likely to try and overturn the changes to the Nuclear Safeguards Bill when it returns to the Commons.
Euratom, an association which is legally separate from the EU but governed by the EU’s institutions, covers issues such as the transport of radioactive materials, including those used in medical treatments, or in nuclear power stations.
The government has said it wants to establish a new domestic nuclear regime as well as negotiate a nuclear agreement with the EU once 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 said the government was “on track” to secure agreements with the United States, Japan and other nuclear partners after Brexit and anything that risked disrupting this process was counter-productive.
But ex-Labour cabinet minister Lord Hutton, who is chair of the Nuclear Industry Association, said remaining in Euratom should be kept alive as a “back-up” option in case other arrangements cannot be negotiated in time.
“I don’t think any of us should take a gamble or a risk with the energy security of our country,” he said.
Some Tory MPs have urged ministers to seek associate membership of Euratom amid concerns that patients’ cancer therapies could be delayed if isotopes can’t be brought in to the UK without a replacement agreement.
Lord Henley said ministers had agreed to provide quarterly reports to Parliament on the Euratom negotiations.
But the government, which does not have a majority in the Upper Chamber, was defeated by 50 votes on this issue after cross-benchers joined forces with Labour and Lib Dem peers to insist on specific assurances over research and development collaboration and the movement of qualifying nuclear material.