Pauline Hanson has sensationally reneged on her promise to support a company tax cut in a move that likely scuttles Malcolm Turnbull's beleaguered policy once and for all.

Labor pounced on One Nation's stunning backflip and said it showed Senator Hanson could not be trusted - even though her latest position is in line with the opposition's.

The government wants to cut the company tax rate from 30 per cent to 25 per cent but has struggled to find support in the Senate. It had been thrown a lifeline in March when Senator Hanson revoked her initial opposition to the plan, in exchange for a $60 million apprenticeship plan.

Finance Minister Mathias Cormann, who has been negotiating with the crossbench, had been hopeful of clinching two more votes needed, especially after announcing a widely-welcomed new tax on the digital giants like Google and Facebook.