Around 40 British nationals are believed to have been detained by Israeli authorities following the assault on a Gaza-bound aid flotilla, the Foreign Office said.
Consulate officials are "urgently" trying to establish the exact number of Britons being held and are seeking immediate access to them.
It was confirmed earlier that one Briton was injured in the attack and had received medical treatment.
A spokeswoman for the Foreign Office said: "We believe there are roughly 40 British nationals involved in the flotilla though, given the confusing situation, we are urgently trying to establish the actual figure as well as making sure we have consular access to them all."
It is believed that the Britons are among hundreds of activists being detained in southern Israel.
The Israeli authorities have said that, of the 679 people taken off the flotilla, 50 had been taken to Israel's international airport for deportation.
The rest had refused to identify themselves.
Campaign groups in the UK said they believed that mobile phones had been confiscated by Israeli guards, leaving relatives in the UK facing an anxious wait for news.
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