The former head of MI6 has suggested Brexit might not happen as he warned ministers that leaving the EU could diminish the UK's diplomatic firepower.
Sir John Sawers told a House of Lords committee that leaving the bloc could make it more difficult for Britain to deal with "difficult" countries like Russia.
Tellingly, he hinted that the decision to leave the EU could be overturned, despite both Labour and the Government pledging to see through the Article 50 process.
In a pre-prepared statement Sir John told peers: "The vehicle through which we have conducted sanctions regimes for the last 20 or so years has been the European Union.
"So clearly the Brexit, assuming it goes ahead, will have an impact on how we take part in coercive foreign policy strategies and it's not just the compiling of sanctions regimes, it's our whole approach to how do we deal with difficult countries like Russia, Iran or North Korea or how we deal with terrorist organisations like Al Qaeda or ISIS.
"Sanctions tend to be part of a wider strategy and we need to think about how the United Kingdom is going to play a role once the centre-piece of our policy coordination and application of policy and sanctions is no longer available to us."
Sir John was speaking to a House of Lords committee on EU sanctions, where he warned that UK action against Russia had been ineffective because it was taken without support from other nations.
Although the sanctions may have made ministers "feel better" in the aftermath of the death of Alexander Litvinenko on British soil, in reality they did little to curb Russia because they were not imposed unilaterally, he said.
Sir John is not the first senior figure in politics to suggest Brexit could be stopped or reversed.
Former Liberal Democrat leader Nick Clegg has been vocal in his claim that if public perception of the decision to leave shifts, a reversal could take place.
Tony Blair, the former Labour Prime Minister, has said the same thing.
Senior lawyers and politicians have yet to find a legal way to reverse the decision however, as once Article 50 has been triggered there does not appear to be a way to undo it.
Brussels sources have suggested that political will from the other 27 EU nations could halt the process if the UK were to change its mind before it officially leaves the bloc.
But both Theresa May and Jeremy Corbyn have promised to see the process through to the end and the House of Commons has officially voted to back Article 50 and the timetable for leaving the union.