No-oral-modification clause in a commercial contract — can the parties vary orally?
Counterparty challenges arbitration agreement validity under NY Convention Art. II — SIAC-seated dispute
Your no-oral-modification clause is prima facie enforceable under English law following MWB Business Exchange Centres Ltd v Rock Advertising Ltd [2018] UKSC 24▣ binding. The estoppel exception survives, narrowly. Your client is partially right: estoppel by convention can override a NOM clause where the parties have acted on a shared assumption of variation — but routine oral discussion will not cross that threshold.
The rule in MWB
The Supreme Court in MWB v Rock Advertising [2018] UKSC 24▣ binding restored orthodoxy: NOM clauses mean what they say. An oral variation made in breach of a NOM clause is not effective as a contractual variation.
The estoppel exception
Lord Sumption expressly preserved the possibility that estoppel may prevent a party from relying on a NOM clause — but only where some words or conduct unequivocally represented that the variation was valid. Compare Globe Motors Inc v TRW Lucas Varity [2016] EWCA Civ 396◇ persuasive obiter, Beatson LJ.
Applied to your facts
Routine oral discussion will not cross the threshold. The client would need to show: (i) a shared assumption of variation per Tinkler v HMRC [2021] UKSC 39▣ binding, (ii) reliance on that assumption, and (iii) detriment. Your colleague may be conflating contractual variation with equitable estoppel — but MWB remains the definitive authority. Earlier authority in Amherst v James Walker Goldsmith [1983] Ch 305⚠ verify citation pre-MWB, historical is no longer determinative.
The challenge is weak but not frivolous. Under Sulamérica CIA Nacional de Seguros SA v Enesa Engenharia SA [2012] EWCA Civ 638▣ binding, the law governing the arbitration agreement is presumed to follow the substantive contract — here, English law. Authority to bind is therefore an English-law question, while the seat (Singapore) governs procedural validity. BCY v BCZ [2017] SGHC 53◇ persuasive applies the same logic in Singapore. Expect the tribunal to uphold jurisdiction under kompetenz-kompetenz (SIAC Rule 28.2) and continue.
Separability and kompetenz-kompetenz
The arbitration agreement is severable from the main contract: even if the underlying contract is voidable for want of authority, the arbitration clause survives unless the same defect attacks it directly. The tribunal decides its own jurisdiction first per Fiona Trust & Holding Corp v Privalov [2007] UKHL 40▣ binding. Only after a tribunal ruling can the Singapore High Court review under s.10 IAA.
Authority to bind under English law
Apparent authority will likely be made out. Under Freeman & Lockyer v Buckhurst Park Properties [1964] 2 QB 480▣ binding, the test is whether the principal held the agent out as authorised. If the signatory was a director or held a senior commercial title, the counterparty's challenge collapses on the apparent-authority limb — even without actual authority.
NY Convention Art. II — limited scope here
Art. II(2) requires an "agreement in writing" — a low bar that the SIAC clause clearly meets. Art. II is more relevant at the enforcement stage; raising it pre-award is rare and rarely succeeds. Compare Dallah Real Estate v Pakistan [2010] UKSC 46◇ persuasive enforcement-stage challenge succeeded on different facts.