The UK National Security and Investment Act in Practice

The National Security and Investment Act came into force in early 2022 with significant uncertainty about how the new Investment Security Unit would interpret its broad mandate. Three years of practice has produced a recognizable rhythm: most filings clear within the statutory thirty-working-day initial review period; a meaningful minority enter the call-in / assessment phase; and a small but consequential number receive substantive remedies or, very rarely, prohibition.

The mandatory regime - covering acquisitions of qualifying entities active in seventeen specified sectors - accounts for the bulk of filings. The sector definitions have proven broader in practice than many practitioners initially read them, particularly for advanced materials, computing hardware, and certain energy-adjacent activities. The default assumption when in doubt should be 'file and explain,' not 'analyze and decide not to file.' The penalties for failure to file a mandatory transaction are significant, and the call-in power gives the Unit five years to revisit non-notified transactions that should have been notified.

The voluntary notification regime is more discretionary, and the practice has been to file voluntarily in three categories: transactions that touch sensitive sectors not on the mandatory list, transactions involving acquirors from jurisdictions of heightened concern, and transactions where the parties want certainty before substantial integration investment. The cost of a voluntary filing is real (the timing impact, the disclosure burden, the modest risk of conditions) but the alternative - a call-in twelve or twenty-four months after closing, when integration is well advanced - is materially worse.

The practical patterns over the last three years are worth flagging. First, the Unit responds well to early engagement. Pre-notification discussions, while informal, have become a meaningful part of the practice for complex transactions. Second, the documentary record matters: the Unit will request strategic plans, board materials, and integration plans, and those documents should be drafted with the understanding that a UK government reader will see them. Third, the remedies the Unit has imposed have followed a recognizable pattern - typically governance commitments, technology safeguards, supply-continuity undertakings, and personnel-related restrictions - and counsel can negotiate from a baseline of published precedent.

The interaction with other regulatory regimes is increasingly important. UK NSI runs in parallel with EU Member State FDI regimes, with CFIUS in the US, and with merger control in multiple jurisdictions. The transactions that move smoothly are the ones where the regulatory workstreams are coordinated centrally, the substantive narratives are consistent across jurisdictions, and the timing is sequenced with awareness of where the long pole will fall.

Two specific points are worth flagging for 2025 deals. First, the Unit has shown increasing interest in upstream investors - not just the immediate acquiror but the limited partners and beneficial owners behind it. The investor due diligence package now needs to address the upstream chain with more rigor than was previously typical, particularly for sponsor-led acquisitions with diverse LP bases. Second, the Unit's coordination with allied regulators (notably CFIUS and the major EU FDI authorities) appears to be deepening, and parties should expect substantive consistency in how a single transaction is reviewed across jurisdictions.

Our practical guidance is straightforward. For mandatory-sector transactions, file. For voluntary transactions in adjacent sectors or with sensitive investor profiles, file. Engage the Unit early in complex matters. Treat the documentary record as a persuasion document. And coordinate with the parallel regulatory workstreams from the start of the deal, not from signing.

What we are watching

We will return to this topic across the coming quarter. If you are actively negotiating a transaction where these issues are live, we'd welcome a confidential conversation.

Three takeaways

• The market is settling, but the diligence bar is rising.
• Preparation, not posture, is the source of speed.
• The right structure can move price more than another round of negotiation.