Parliamentarians in Equatorial Guinea have been banned from travelling outside the country unless the trip is authorised by Vice-President Teodorin Nguema Obiang, the son of the president.

A reporter with the AFP news agency obtained the letter which was sent last month to lawmakers and it says the measure has been put in place on grounds of national interest.

In June a similar ban was imposed on all civil servants.

The restriction is suspected to be as a result of an attempt last year to overthrow President Teodoro Obiang Nguema - Africa's longest serving leader.

The government of Equatorial Guinea said the coup attempt was instigated by civil servants who had travelled to Europe.