Quick answer

It depends on the form and the reason: under the ICE 5th Edition interest on undercertification is due only where the engineer wholly failed to certify or undercertified through a misapplication or misunderstanding of the contract, not an honest lower valuation, while the ICE 6th and 7th Editions make interest payable even for good-faith undercertification, and under the JCT forms a court may award interest running from the date on which the certificate ought to have been issued

It depends on the form and the reason. Under the ICE 5th Edition, clause 60(6), the courts have split. The Scottish decisions in Nash Dredging v Kestrel Maritime and Hall and Tawse v Strathclyde Regional Council held there was no failure to certify where the engineer honestly certified what he believed was due, while Morgan Grenfell Ltd v Seven Seas Dredging Ltd allowed interest even for good-faith undercertification. The English position was settled the other way in Secretary of State for Transport v Birse-Farr Joint Venture: interest is due only where the undercertification results from a misapplication or misunderstanding of the contract by the engineer, not an honest lower valuation, and the burden is on the contractor. The ICE 6th and 7th Editions, clause 60(7), make interest payable on undercertification even in good faith. Under the JCT forms the courts have allowed interest running from the date on which the certificate ought to have been issued, as in Amec Building v Cadmus Investment Co Ltd, and in Charles Brand Ltd v Orkney Islands Council interest was payable for a period during which the employer had the use of money withheld under a set-off that later fell away.