William Hague, foreign secretary, four former foreign secretaries and three former Foreign Office ministers have signed a letter to the Times opposing the alternative vote:
"Those of us who have represented Britain internationally know that one of the many reasons why we have always punched above our weight is our simple and straightforward voting system, a system that everyone can understand, because it gives one person, one vote.
"Democracies all across the world have been founded on the example of our voting system. Today, billions of people elect their representatives through the system of one person, one vote.
"It took many centuries for the principle of one person, one vote to become enshrined in our democracy. And now that it is there, we believe it would be a grave error to abandon this principle and replace it with a voting system that is more complex, more confusing, more costly and more unfair."