(Reuters) - Egyptian investigators said on Thursday they had raided 10 foreign exchange bureaus in Cairo after an inquiry found they were illegally trading foreign currency at black market rates.

A statement from the Public Funds Investigation unit said it had also targeted some bureaus in Alexandria and Cairo for trading in currencies after the central bank had revoked their licenses.

The central bank has sought to crack down on a burgeoning black market for dollars, closing bureaus and revoking the licenses of those found to be trading far beyond the official rate of 8.78 Egyptian pounds to the dollar.

The Egyptian pound strengthened to 12.50 to 12.70 to the dollar on the black market on Thursday from unprecedented levels of 13 to 13.25 on Monday.

The central bank blames speculation by black market traders for increasing downward pressure on the currency that it can ill-afford to defend.

Egypt's foreign reserves have shrunk from about $36 billion before the 2011 uprising to about $17.5 billion in June.

(Reporting by Ahmed Shalaby; writing by Lila Hassan; editing by Lin Noueihed/Mark Heinrich)