A complaint letter is a formal written request for a company, organisation, or public body to acknowledge a problem and put it right. You should use one when you have exhausted informal attempts to resolve an issue — for example, after a failed phone call or in-store complaint. A well-structured written complaint creates a paper trail, signals seriousness, and is often required before you can escalate to a regulator or ombudsman. Common situations include faulty goods, poor service, billing errors, NHS treatment concerns, and problems with a council or utility provider.

An effective complaint letter is clear, factual, and professional in tone. It should state exactly what happened and when, reference any relevant consumer rights (such as the Consumer Rights Act 2015 for goods and services), specify what resolution you want — a refund, repair, apology, or explanation — and set a reasonable deadline for a response, typically 14 days. LetterDeck structures your letter around these principles automatically, so you do not need to know the law yourself.

LetterDeck's complaint generator supports letters to private companies (retailers, telecoms, energy suppliers, insurers), NHS trusts and GP surgeries, local councils, letting agents, and other public bodies. For each, the AI applies the appropriate tone and references the correct UK regulatory body — for example, the Financial Ombudsman for financial complaints, Ofgem for energy, and the Parliamentary and Health Service Ombudsman for NHS matters.

If a company does not respond within your stated deadline, your written complaint becomes evidence of an unresolved dispute. You can then escalate to the relevant ombudsman or regulator, raise a chargeback with your bank (for card payments), or pursue a small claims court claim. LetterDeck can also help you draft follow-up escalation letters.