By Bridget Kendall
Diplomatic correspondent, BBC News
This is not the first diplomatic incident to involve British crew members being taken by Iran.
In 2007, 15 British sailors were arrested and detained for 12 days in the Shatt al Arab waterway between Iraq and Iran.
And in 2004 eight British servicemen were arrested and accused of spying in the same area.
In both cases, the captured sailors were paraded on TV, but eventually pardoned and released by the Iranians, amid tense exchanges between Tehran and London.
Since then relations have become even more strained. Iran accused Britain of helping to fuel last summer's street protests.
And Britain has kept up the pressure to try to get Iran to freeze its nuclear programme.
Only this weekend David Miliband condemned Iran's latest plan to build more uranium enrichment plants. Not an auspicious context for a quick resolution to the incident.
Even so, this incident is different.
These latest British crew members are not military personnel, but competitive sailors, on their way to take part in a yacht race.
They were stopped not in the sensitive disputed region between Iraq and Iran, but lower down in the Gulf.
And this time there is no disagreement about what may have happened - everyone seems to agree that they may have mistakenly strayed into Iranian waters.
So will Iran release them swiftly? Clearly the Foreign Office hoped this was what would happen if they kept the incident out of the public eye.
So far Iran has said nothing publicly and apparently given no clear response through diplomatic channels either. This may be because the Eid holiday has made contact with senior Iranian officials difficult.
But the next few days will be critical.
Britain's Foreign Secretary David Miliband has let it be known he wants to speak to the Iranian Foreign Minister directly. The ball is in Iran's court.
Either Tehran can decide to play the incident down and let the sailors go, or it could turn this into a full blown diplomatic crisis.