The UK is "deeply concerned" about a Briton missing in Egypt for 10 days and has demanded to know if he is being held by the authorities, a Foreign Office minister said.
Alistair Burt said Hisham Morsi was last seen being removed from Tahrir Square on January 31 but it remained unclear by whom and whether he was arrested.
Mr Burt raised the case with the Egyptian ambassador on Wednesday said it was vital Cairo met a promise to cease "arresting and harassing journalists, foreigners and members of the opposition".
"We had been given public and private assurances by the Egyptian government that no activists remained in detention," Mr Burt said in a statement.
"In light of this, I call on the Egyptian authorities to urgently clarify whether Mr Morsi has been arrested, and if so, to inform our embassy of his whereabouts and provide our embassy with full and immediate consular access.
"I raised this case with the Egyptian Ambassador, and our embassy in Cairo continues to make representations to the authorities in Egypt.
The Egyptian government has committed itself to releasing all detained protesters, not to arrest anyone for expressing ideas and opinions and to stop arresting and harassing journalists, foreigners and members of the opposition.
"It is important that these commitments are implemented immediately."
A Foreign Office spokeswoman said Mr Morsi is of dual British and Egyptian nationality.
It is believed he is about 50 years old and lives outside the UK, "probably in Cairo", said the Foreign Office. His family, also believed to live outside Britain, are receiving consular assistance.