Switching stages: how to vary your commercial contracts without losing the plot

Commercial contracts rarely remain static. Over the life of a supply agreement, outsourcing arrangement or services contract, the parties may need to adjust scope, pricing, timelines or operational detail.

However, the way those changes are introduced can determine whether they are legally binding or dangerously uncertain. This article considers the requirements for valid variation under English law, the risks of informal side deals and practical steps for managing change effectively.

The legal framework for variation

Under English common law, a contract variation is itself a contract. It requires offer, acceptance, consideration and an intention to create legal relations. Where one party agrees to confer a new benefit without receiving anything in return, the variation may fail due to a lack of consideration unless it’s executed as a deed.

Many commercial agreements go further by including a ‘no oral modification’ (NOM) clause, requiring all variations to be made in writing and signed by authorised representatives. The Supreme Court has ruled that NOM clauses are enforceable under English law. Where such a clause exists, an oral or informal agreement to vary the contract, however clearly intended by the parties, may be wholly ineffective.

The risks of informal variation

Despite the clarity of the legal position, commercial reality often diverges from contractual process. Variations are agreed by email, in telephone conversations or simply through a course of conduct that neither party formally documents. Such changes risk being unenforceable where the contract contains a NOM clause.

A party that has relied on an informal promise to its detriment may invoke estoppel as a defence, but the boundaries of this remain uncertain and parties shouldn’t rely on estoppel as a substitute for proper process.

Informal variations also create evidential difficulties. When a dispute arises, reconstructing what was agreed, and by whom, from a patchwork of emails and recollections is both costly and unreliable. The result is often an uncomfortable gap between the written contract and actual performance on the ground.

Best practice for managing change control

For long-term supply and outsourcing contracts, a well-drafted change control procedure (CCP) is essential. Parties should consider the following principles:

• Include a CCP in the contract from the outset, specifying who has authority to approve variations and at what level of seniority or financial threshold
• Require all variations to be documented using a standard change order form, signed by authorised representatives of both parties before any change takes effect
• Assess the impact of each proposed change on pricing, timelines and technical specifications before approval is granted
• Maintain a central variation register or log so the cumulative effect of changes can be tracked and audited
• Periodically consolidate agreed variations into an updated version of the agreement, ensuring the contract remains a reliable single source of truth
• Train commercial and operational teams on the importance of following the formal process, as change control is only effective if relevant personnel understand and observe it.

Conclusion

Contract variation is inevitable in long-term commercial relationships, but it doesn’t need to be a source of risk. By understanding the legal requirements, respecting NOM clauses and embedding robust change control mechanisms, parties can ensure their agreements evolve in a controlled, enforceable and transparent manner. The time to get this right is at the drafting stage, not when a dispute has already arisen.