NIG is set to become the seventh insurer to join imarket.
Market sources said the insurer, part of Royal Bank of Scotland Insurance, would pay "significantly less than £1m" to join.
An NIG spokesman said: "We are very keen to get involved with imarket and are in active discussions to make this a reality."
Membership of the imarket online trading platform currently consists of Norwich Union, Royal & SunAlliance, AXA, Allianz Cornhill, Zurich and Groupama.
One source said: "Though the six current members already control about 60%-70% of the market, Polaris see this as a major vote of confidence and proof that the group of six is not a closed shop."
A further group of insurers, understood to include Fortis and MMA, are in early negotiations to join imarket.
