Where a defendant obtains a benefit as a result of an unlawful demand the ground of duress might be engaged.38 Since the restitutionary claim in respect of ultra vires payments to a public authority is by definition unlawful, the claimant will be able to obtain restitution on the ground of duress of the person if the payment was caused by the public authority threatening to restrain, or actually restraining, the claimant, or duress of goods if the claimant threatened to seize, or actually seized, the claimant’s property. Alternatively, restitution could be grounded on economic duress if the defendant threatened to breach a contract, commit a tort, or do some other unlawful act. So, for example, the claimant might be able to recover money from the defendant on the ground of economic duress if the claimant paid the money as the result of an unlawful threat by the defendant to withdraw a licence if the money was not paid. But the most likely form of duress to be applicable in respect of restitutionary claims against public authorities is duress of goods, since often the authority will have the sanction to seize the claimant’s goods if the money demanded is not paid. Such a threat is unlawful because the authority is not authorized to seize goods in respect of a debt which is not lawfully due. Where, however, the authority threatens to institute legal proceedings to recover the money if it is not paid, this does not constitute duress because the claimant was given an opportunity to dispute the legality of the demand in legal proceedings. Consequently, payment in response to such a threat is characterized as voluntary and is not recoverable on the ground of duress.
-  This would also be covered by the ground of restitution known as ‘extortion by colour of office’. Seebelow.
-  See Maskell v Horner  3 KB 106. Enonchong suggests in ‘Restitution from Public Authorities’, 90,that this may also be relevant to claims for restitution of payments made to utilities which have been privatized.
-  William Whiteley Ltd v R (1910) 101 LT 741.