Microsoft has added Anthropic's Claude models to Microsoft 365 Copilot. What began as an option inside the Researcher agent and Copilot Studio has expanded considerably: in May 2026, Claude became the default model for Copilot in Excel and PowerPoint across most of Microsoft's commercial cloud, with Word following over the summer [1].
For most of the world, that happened automatically. For UK tenants, it did not happen at all, and that asymmetry is the story.
For tenants in the UK, EU and EFTA, Anthropic models are switched off by default. Nothing changes until a Global Administrator explicitly opts in through the Microsoft 365 admin centre, using the setting Microsoft introduced in April 2026 for AI providers operating as Microsoft subprocessors [1] [2].
Two practical points follow. First, the decision sits with whoever holds Global Administrator rights on your tenant, which in many firms is an external IT provider. If you have not made this decision consciously, it has been made for you by the default. Second, the controls are not one single switch: availability differs across Copilot Chat, Researcher and the individual Microsoft 365 apps, and the settings have already been reorganised more than once this year [2] [3]. A firm that assumes one toggle covers everything can end up either blocking capability it wanted or allowing processing it has not reviewed.
When Claude is used inside Copilot, the processing happens on Anthropic's infrastructure, outside the Microsoft-managed environment and outside the EU Data Boundary that the rest of Copilot operates within [3] [4]. Anthropic acts as a subprocessor, and Microsoft's documentation is explicit that Anthropic, not Microsoft, stores most inputs and outputs for up to 30 days under its commercial data retention policy [2].
None of that makes Claude unusable for a law firm. It makes enabling it a decision rather than a default, which for UK professional services is exactly the right way round.
Claude's reputation is for long documents and heavier reasoning: the disclosure bundle, the lengthy agreement, the multi-document question that a shorter-context model handles badly. Those are precisely the document shapes legal work produces, which is why the model choice matters more to a law firm than to most businesses. Inside Copilot, that capability arrives in the tools your fee earners already use, rather than as another application to roll out.
We deliberately frame these as questions rather than answers, because they belong with your firm's own compliance officer and data protection lead:
Which client engagements include data residency commitments, and would Claude processing sit within them?
Which Copilot surfaces would Claude be enabled for, and for which teams? Scoped enablement for a defined group is possible and is often the sensible starting point [4].
How will use be recorded and supervised, and how would the firm evidence that supervision if asked?
What does the DPO need documented before the toggle moves: the transfer mechanism, the retention position, and whether an impact assessment is required?
If the firm decides not to enable it, is that decision recorded with its rationale? Declining is as legitimate an outcome as enabling, but both should be deliberate.
Model access is not a strategy. Whether Claude in Copilot is worth enabling for your firm depends on which use cases you would actually run it against, and that depends on your practice areas and where your matter data lives.
We have mapped 46 use cases for Copilot and Claude across UK legal practice; our free calculator filters them to your practice areas and your practice management system, with an illustrative figure against each. Five minutes, no obligation.
And because Nexian runs both sides, the Microsoft 365 environment and the practice and case management layer, the tenant configuration, governance and data questions above are ones we handle every week rather than pass to a third supplier.
Reviewed September 2026.
[1] Microsoft Learn, 'Anthropic models in Microsoft Online Services': the April 2026 admin centre setting, and Anthropic's 30-day commercial data retention position. https://learn.microsoft.com/en-us/microsoft-365/copilot/connect-to-ai-subprocessor
[2] Cloudswitched, on the May 2026 default change (Claude default in Excel and PowerPoint for most commercial tenants; UK/EU/EFTA tenants off by default pending Global Administrator opt-in). https://www.cloudswitched.com/news/microsoft-365-copilot-anthropic-default-may-2026-uk-business-opt-in-guide
[3] Jersey Finance / Prosperity 247, on Claude availability arriving differently across Copilot Chat, Researcher and the Microsoft 365 apps, and processing outside the EU Data Boundary. https://www.jerseyfinance.com/insights/anthropic-comes-to-microsoft-365-copilot-what-it-means-for-uk-and-eu-customers/
[4] The AI Consultancy, 'Claude in M365 Copilot: UK Guide' (verified July 2026 against Microsoft documentation): the two separate tenant settings with opposite defaults, and per-group enablement. https://theaiconsultancy.ai/blog/anthropic-models-microsoft-365-copilot-uk
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Or get in touch with the team if you would like to talk through where your firm is on its technology journey.