Relations between Britain and Iran have always been delicate but are particularly sensitive at the moment because of strains over Iran's nuclear programme and human rights record.
By Bridget Kendall
BBC diplomatic correspondent
For months Britain's relations with Iran have been a difficult balancing act.
British-Iranian relations are under pressure again
British forces in southern Iraq have been under orders, apparently, to keep border tensions with Iran to a minimum.
And nowhere more so than over the Shatt al-Arab waterway, Iraq's main access to the Persian Gulf, and the source of the territorial row that led to an eight year war between Iran and Iraq in the 1980s.
What lies behind Monday's incident is not clear yet but it may well be due to further pressure on coalition forces to do more to stop cross border smuggling into Iraq and infiltration by suspected terrorists.
If this is a case of a British patrol team straying too far, British diplomats certainly hope they can solve it quickly.
Iran was furious at the criticism directed at it last week by the UN's top nuclear agency, for what was termed a deplorable failure to cooperate fully with UN inspectors.
And a stinging rebuke on Iran's human rights record from the EU this weekend did not help matters.
No-one is suggesting this is a deliberate gesture by Iran of diplomatic retaliation.
But the worry remains that it could all too easily escalate into a more serious crisis, given the strains between Tehran and London at the moment.