Keir Starmer could have to give the green light for US bombers to use the Diego Garcia military base for strikes on Iran.
Donald Trump is keeping the world guessing about whether he will join Israeli attacks on Tehran's nuclear facilities.
The PM has been urging 'de-escalation' and so far UK forces are not thought to have played a direct part in defending Israel from retaliation.
However, Sir Keir might be forced to choose amid speculation that B-2 stealth bombers would use the Chagos Islands airbase if they are brought in to target an enrichment plant deep under a mountain.
There are concerns that would leave UK assets and personnel facing reprisals, with RAF Akrotiri on Cyprus a potential target. More jets have been moved into the region to bolster security.
