Labour's support has slumped to 29 per cent in the latest Roy Morgan poll - a blow to Labour leader David Shearer.

The poll puts National on 51 per cent, on the back of a 4 per cent rise in support since the same poll a fortnight ago.

Shearer has been dogged by rumours about his leadership and Labour has struggled to lift itself out of the polling doldrums.

The poll questioned 824 eligible voters on either land line or mobile phone from July 15 to 28