National has increased its lead over Labour in the latest Roy Morgan Research poll.

The survey released tonight shows National on 53 per cent support, up 2.5 points, and Labour down four points to 29 per cent.

The Labour result is its lowest in Roy Morgan polls this year and the gap between the main parties is 24 points.

"Revelations about inappropriate credit card spending over recent years by many Labour MPs while in government are clearly having an impact on electoral appeal and appear to be behind the fall in the Labour vote," the poll analysis said.

There is little change in support for the minor parties.

Tonight's poll questioned 849 voters between June 21 and July 4.

The margin of error is about plus or minus 3 per cent.