LONDON — Theresa May is in a race against time to wield the full force of EU sanctions against Moscow — the prime suspect in the dramatic poisoning of a Russian former spy on British soil.

She knows that as soon as Britain loses its seat at Brussels’ foreign policy table and begins the post-Brexit transition period, her country could have far less sway in Europe — and, at least in the short term, no unilateral sanctions policy either.

With the U.K. gripped by the police investigation into the poisoning of Sergei Skripal and his daughter Yulia, the British foreign policy establishment's thoughts have turned to retaliation against Moscow, should the Kremlin be implicated when all the facts are known.

The U.K.’s sanctions regime is currently bound by law to the EU’s — Britain has no legal authority to impose its own sanctions or those agreed at the U.N.

A bill is working its way through the British parliament, which, after Brexit, will make the U.K. a free agent in this respect, but during the two-year transition as the country exits the European Union, London will be chained to EU rules without a say in their formulation.

Critics doubt the U.K. will have the clout to lead a unified push once outside of EU structures.

That leaves May with just one year to use the U.K.’s voice while still a member of the EU to push the typically hawkish British line on Russia, one that is only going to become harder if the hand of the Kremlin is detected in the attempted murder of Skripal, who, along with his daughter, remains seriously ill in hospital after poisoning by nerve agent in Salisbury, southern England, last Sunday.

Foreign Secretary Boris Johnson has portrayed the U.K.’s post-Brexit approach to sanctions in characteristic "cake-and-eat-it" terms.

The U.K. will “no longer be compelled to wait for consensus among 28 members of the EU” before imposing sanctions against other countries, he told MPs. However, he added the U.K. will also aim to act in concert with the EU where possible.

“The outcome will be that Britain enjoys both freedom of maneuver and the option of working alongside our European friends,” he said in the House of Commons last month.

Critics doubt the U.K. — which has struggled to wrestle sometimes reluctant European partners into a tougher stance on Russia, even as an EU member — will have the clout to lead a unified push once outside of those structures.

“When we leave the EU, we will lose our seat at the table. In the six-monthly review of the sanctions, we have continued to push for their renewal. How will we exert influence when we have left the EU?” Labour MP Emma Reynolds, a member of the House of Commons Brexit committee, demanded of the foreign secretary in an urgent parliamentary debate called in response to the Salisbury poisoning.

To guarantee its voice is still heard, the U.K. is pushing for what officials call a common security partnership framework, which could be fast-tracked into existence during the transition period rather than leaving the U.K. in limbo until it fully separates from the EU in 2021.

“We will get control of sanctions once we’ve left but the question is how much we can set up a framework that allows cooperation,” one senior government official said.

Crispin Blunt, the Conservative former chair of the House of Commons foreign affairs committee, said the transition should be a moment to examine the basis of this relationship and is urging the government to push for observer status on the EU's Political and Security Committee — a move that was notably absent from May’s recent Munich speech on the post-Brexit security partnership.

“It’s obviously in the EU’s interest if they can coordinate their position with us … There’s an agonized debate within the EU around sanctions on Russia. Quite where that debate will land when it’s EU27 rather than EU28 remains to be seen,” he said.

Others doubt whether the EU’s position would alter with or without the British in the room. Julian Lewis, chair of the House of Commons defense committee, but speaking in personal capacity, said it would be “inconceivable” that the EU would use Salisbury as a pretext to tighten their stance “without the closest consultation with the country where the attack took place, whatever the constitutional requirements of the transition phase.”

“On the other hand, if the EU decided it was in their interests to do nothing further, then our continued membership would have been unlikely to make much difference to that decision,” he said.