On 7 August 2026 the UK Health and Safety Executive notified the World Trade Organization of a proposal to update the Great Britain Mandatory Classification and Labelling list, the GB MCL list, under the GB CLP Regulation. The proposal introduces new and revised harmonised classifications for 20 hazardous substances and, in one respect that goes beyond the individual entries, a new rule for mixtures containing boron compounds.

For companies placing chemicals on the GB market, the notification is the point at which a change becomes plannable. It is published, dated and open for comment, and it comes with a compliance horizon that leaves enough room to act deliberately rather than in a rush.

What the GB MCL list does

The GB MCL list is the Great Britain equivalent of the harmonised classification and labelling list in Annex VI of the EU CLP Regulation. An entry in the list is mandatory. Where a substance appears there, the supplier must apply at least that classification, together with the associated hazard statements, signal words, pictograms and any specific concentration limits or M factors, regardless of the supplier's own assessment of the data. The classification of the substance then flows through into the classification of every mixture that contains it above the relevant threshold.

Since the end of the transition period, GB CLP has been maintained separately from EU CLP. Entries are added on the basis of Agency Opinions prepared by HSE in its role as the GB Agency, followed by a decision by the Secretary of State. The two systems still overlap substantially, but they no longer move in step.

What is being proposed

The proposal covers 20 substances. Named entries include Bronopol, a preservative widely used in cosmetics, personal care products, detergents and industrial water treatment, Thymol and Eugenol, both of which occur in biocidal, flavouring and fragrance applications, Piperonal, and a group of boron compounds including the naturally occurring borates ulexite and colemanite.

For the substances concerned, the proposed changes affect the following elements.

1. Hazard classes and hazard categories, meaning the substance may be classified for a hazard it did not previously carry, or moved into a different category within a hazard class.
2. Hazard statements, the H statements that appear on the label and in section 2 of the safety data sheet.
3. Specific concentration limits, which determine the concentration at which a substance triggers classification of a mixture. A lowered SCL can bring mixtures into scope that were previously below the generic threshold.
4. M factors, which multiply the contribution of a substance to the aquatic hazard classification of a mixture and can therefore change environmental classification disproportionately.
5. Labelling elements, including pictograms and signal words, which is where the change becomes visible on the product and in the artwork.

The boron mixture rule

The most consequential element of the proposal is not tied to a single substance. Under the proposed rule, a mixture would have to be classified as a reproductive toxicant where the combined concentration of relevant boron compounds classified for reproductive toxicity reaches 0.3 percent or more.

This is an additivity rule, and it changes the nature of the assessment. A formulator who checks each boron containing raw material individually against its threshold may conclude that no classification is triggered, while the sum of the contributions crosses the 0.3 percent limit. Boric acid, borax and related borates appear in a wide range of formulations, often in modest quantities and often from more than one raw material, including as components of biocidal preservative systems or as buffers rather than as headline ingredients.

A reproductive toxicity classification is not a cosmetic change. It brings a health hazard pictogram and a danger signal word, it affects the way the product may be supplied and to whom, it feeds into downstream duties under workplace legislation, and in many procurement environments it affects whether a product remains acceptable to the customer at all.

Key dates

1. Notification to the WTO on 7 August 2026, which opens the formal comment period.
2. Comment deadline on 6 October 2026. This is the window in which industry can raise technical or practical concerns, including on the feasibility of the proposed limits.
3. Expected adoption in the first quarter of 2027.
4. Voluntary application from the first quarter of 2027, meaning suppliers may apply the new classifications as soon as they are adopted.
5. Mandatory compliance from the third quarter of 2028.

The two year gap between adoption and mandatory compliance is the working period, not a waiting period. Label artwork, packaging stock, safety data sheets in every required language and version, and customer facing documentation all need to be aligned before the deadline rather than at it.

What this means in practice

The proposal reaches further than the list of 20 substances suggests, because harmonised classifications propagate into mixtures. A company that supplies none of these substances as such may still supply dozens of formulations that contain them.

The practical impact typically appears in five places.

1. Product classification, which has to be recalculated for every mixture containing an affected substance, taking revised SCLs and M factors into account.
2. Formulations, where a recipe adjustment may be a cheaper answer than accepting a new classification, particularly where the boron additivity rule is the trigger.
3. Safety data sheets, where sections 2, 3, 15 and 16 will generally need revision, along with any exposure scenarios attached to them.
4. Labels and packaging, including pictograms, signal words, hazard and precautionary statements, and the lead times of your printing and packaging suppliers.
5. Compliance documentation, including poison centre notifications where applicable, product registrations, and the records that support your classification decisions.

Recommended next steps

1. Screen your substance and mixture portfolio for the 20 substances concerned, and separately for all boron compounds, regardless of their current classification status.
2. Run the boron additivity calculation across your formulations, adding the contributions of all relevant boron compounds rather than assessing each raw material on its own.
3. Identify the products where a classification would change, and separate them into those where reformulation is realistic and those where relabelling is the only route.
4. Decide whether to comment by 6 October 2026, either directly or through an industry association, particularly where the proposed limits raise practical difficulties you can evidence.
5. Plan label and artwork changes against your packaging stock, so that existing material is used up rather than written off shortly before the deadline.
6. Compare the GB position with the EU position for each affected product and record where the two diverge, so that the difference is documented rather than discovered.
7. Build the change into your normal product release and change control process instead of running it as a standalone project.

Our assessment

Two aspects of this proposal deserve more attention than they usually get. The first is the boron mixture rule, because it is an additivity rule and additivity rules catch companies that check thresholds substance by substance. The second is the divergence between GB CLP and EU CLP. Maintaining two classification datasets for the same product is manageable if the system is designed for it, and error prone if the GB position is treated as a copy of the EU position with occasional exceptions.

The comment deadline of 6 October 2026 is the near term date. The Q3 2028 compliance date is the one that determines the workload. Companies that begin the screening now will spend the intervening period making decisions, and companies that wait will spend it reprinting labels.

If you would like support in screening your portfolio against the proposed entries, in running the boron additivity assessment, or in updating your safety data sheets and labels for the GB market, we are happy to help.