LONDON, March 5 (Reuters) - Seafarers have the right to refuse to sail on ships passing through the Middle East Gulf, including the Strait of Hormuz, after the threat level for the region was raised to its highest level, the leading labour union and shipping industry groups said on Thursday.Around 300 ships are anchored on both sides of the Strait while the U.S.-led war against Iran escalates. Since February 28, nine ships have been damaged and at least one seafarer has been killed.