The tanker is now at Iran's Bandar Abbas port and all of its 23 crew members will remain on the ship until the probe is over.
The British-flagged tanker Stena Impero was in an accident with a fishing boat before being detained on Friday, Iran's Fars news agency reported on Saturday, quoting an official.
Iran says all 23 crew seized on the tanker are now at Bandar Abbas port and will remain on the vessel until the end of an investigation, according to Fars.
"It got involved in an accident with an Iranian fishing boat... When the boat sent a distress call, the British-flagged ship ignored it," said the head of Ports and Maritime Organisation in southern Hormozgan province, Allahmorad Afifipour.
"The tanker is now at Iran's Bandar Abbas port and all of its 23 crew members will remain on the ship until the probe is over."
Britain said earlier it was urgently seeking information about the Stena Impero, which had been heading to a port in Saudi Arabia and suddenly changed course after passing through the strait at the mouth of the Gulf.