The impact of Brexit on the UK's ability to achieve its foreign policy goals requires urgent consideration, peers said as they warned the Government's approach to sanctions co-operation with the remaining 27 members of the European Union is "untested".
The House of Lords European Union Committee said that sanctions were most effective when implemented on a multilateral basis, but the UK could lose influence over the EU after leaving the bloc.
"The influence of the UK on the sanctions policy of its international partners will depend on the extent to which it is able to retain its authority and leadership on key foreign policy dossiers after Brexit," the peers said.
"Further consideration of the impact of leaving the EU on the UK's ability to pursue and achieve its foreign policy objectives will be urgently required."
Setting out how the UK could choose to impose sanctions after Brexit, the committee noted that Norway and Switzerland align themselves with the EU regime.
While this would preserve the current unity, it would require the UK to implement decisions taken by the remaining 27 EU members without any say over the design of the sanctions.
Following the US approach of "informal engagement" with the EU on sanctions could be valuable but "it is no substitute for the influence that can be exercised through formal inclusion in EU meetings".
The Government's preferred approach is an "unprecedented" UK-EU partnership on sanctions.
"The UK has some leverage in that it currently plays a leading role in developing EU sanctions policy, is most active in proposing individuals and entities to be listed, and is home to the largest international financial centre in the bloc," the report said.
"But we note that the Government's approach is untested and it is not yet clear what its proposed arrangements would involve.
"Future co-operation could also be limited by the UK's new legal framework for sanctions and its post-Brexit position outside the EU's single market and customs union.
"More broadly, the extent to which the UK and the EU co-operate on sanctions will depend on their future relationship in the wider foreign policy arena. This needs urgent consideration."
A Foreign Office spokesman said: "The committee is right to acknowledge that we want to continue working closely with EU and international partners on sanctions post-Brexit.
"Sanctions are most effective when delivered jointly by a broad coalition of countries."
In a paper published in September, the Government suggested there should be "regular close consultations on foreign and security policy issues, with the option to agree joint positions on foreign policy issues" which "could include co-operation on sanctions listings, including by sharing information and aligning policy where appropriate".
The spokesman added: "As set out previously, we are committed to co-operating closely with our European allies on foreign and security policy questions.
"We want a unique partnership on CFSP (common foreign and security policy) - including on sanctions.
"UK-EU talks on the future partnership have not yet begun so it is too early to comment on the precise arrangements by which this will be done and therefore commit to a 'political forum' as recommended by the committee. But this report is a welcome contribution to the debate."