Mr Burnham said only he would be able to prevent “civil war” by uniting the left and right wings of the Labour Party after a bitter leadership campaign.
Meanwhile Ms Cooper’s campaign cited internal polling showing only she will be able to defeat Jeremy Corbyn for the party’s leadership.
Former cabinet minister Liam Byrne, one of Ms Cooper’s supporters, said the campaign’s internal polling showed she was “clearly on course to beat Jeremy in the final round”.