Standard and Poor's on Tuesday downgraded the credit ratings of four Greek banks to "selective default", or SD, following the imposition of limits on cash withdrawals and foreign transfers.

"The downgrades to 'SD' follow the measures introduced by the Greek government", including "limits to deposit withdrawals, the closure of bank branches for a full working week, and the prohibition of money transfers out of Greece unless authorised by the Greek Ministry of Finance," the ratings agency said in a statement.