British Prime Minister David Cameron has got off to a tough start in the EU referendum campaign, scoring a victory in Brussels but then seeing six ministers and dozens of his Conservative MPs including Mayor of London Boris Johnson back Brexit, experts said. The charismatic Johnson came out of his house in north London on Sunday and declared that he would be supporting a "Leave" vote in the June 23 referendum on EU membership, despite an appeal from Cameron to change his mind. Anand Menon, a European politics professor at King's College London, told AFP that Johnson's endorsement would give the Brexit campaign "traction" but it was not yet clear how prominent his role would be.