Hamas, which controls the Gaza Strip, announced freelance journalist Paul Martin's arrest on Sunday, saying it had received "confessions" about security crimes. It gave no details about the allegations.
A spokesman for the Hamas-run Interior Ministry said Martin was being held under a 15-day detention order issued by the attorney-general in the Gaza Strip.
"He may be released or the 15 days may be extended, pending (the results) of the investigation," said the spokesman, Ehab al-Ghsain.
A spokeswoman for the British consul-general in Jerusalem, who oversees affairs in the West Bank and Gaza Strip said British diplomats were helping Martin in Gaza and were in touch with his family.
"We are very concerned about the situation," she said.
Ghsain said that a Palestinian acting on behalf of the British consulate visited Martin on Sunday and the journalist chose a Palestinian lawyer to represent him.
Like other Western nations, Britain has long rejected any official dealings with Hamas over the Islamists' refusal to recognise Israel and renounce violence.
London does not recognise the government Hamas established in Gaza in 2007 after it broke violently with the Palestinian Authority of West Bank-based Palestinian President Mahmoud Abbas.
However, Western diplomats are regular visitors to the Gaza Strip, where much of the 1.5 million population is dependent on aid from the United Nations and other bodies.
Journalists accredited by the Israeli government are among the few foreigners allowed by Israel to enter the Gaza Strip.
Britain, like other Western nations, advises its citizens not to go there.
Human rights groups have criticized both Hamas and Abbas's Palestinian Authority for detaining journalists and placing other curbs on media freedoms.
