The Post Office may have underpaid more than £100m in tax by deducting payments to victims of the Horizon scandal from its profits, say tax experts.
Claiming tax relief in this way could count as a breach of the law and could mean it is at risk of insolvency, said Dan Neidle, of Tax Policy Associates.
The company also may have overpaid senior executives, he says.
The Post Office said its financial information was "appropriate and accurate".