Two skiffs followed a merchant vessel off the coast of Iran on Wednesday, United Kingdom Maritime Trade Operations (UKMTO) warned.
The green vessels came within 200 metres of the unnamed ship at 11.55am London time, the body said in an advisory note on its website.
It cited a report from the ship.
Both skiffs had two people on board.
Tensions have been high in the region following the tit-for-tat seizures of Iranian and British tankers this summer.
