"Even if the ship had somehow strayed into Iranian waters, Beckett said, "under international law, warships have sovereign immunity in the territorial sea of other states."

"The very most Iran would've been entitled to do if they considered our boats were breeching the rules on innocent passage would've been to require the ship to leave their territorial waters immediately," the foreign secretary explained.

On Monday, hard-liners in Iran urged the government to charge the Britons with espionage and put them on trial."

http://www.cnn.com/2007/WORLD/meast/03/28/iran.uk.sailors/index.html