UK Dieselgate Group Litigation: Multi-billion Mass Tort

The largest consumer group action in English legal history. About 1.6 million owners of Euro 5 and Euro 6b diesel cars allege that 14 manufacturer groups fitted prohibited defeat devices in breach of EU Regulation 715/2007 and seek damages in contract, statute and tort. A ten week test trial against five lead manufacturers ran from October to December 2025.

LATEST DEVELOPMENT · JUL 10, 2026

On 10 July 2026, Cockerill LJ handed down a 369 page judgment finding prohibited defeat devices in one Mercedes vehicle and two Peugeot-Citroën vehicles while dismissing the allegations against Ford, Renault and Nissan. Both sides have said they are considering appeals.

BACKGROUND

This is the largest group action in English legal history, consolidating claims by roughly 1.6 million owners of Euro 5 and Euro 6b diesel vehicles against fourteen manufacturer groups under a series of Group Litigation Orders known collectively as the Pan-NOx (or Emissions) Group Litigation. Claimants allege the manufacturers fitted prohibited defeat devices that caused vehicles to pass regulatory emissions tests while emitting far higher nitrogen oxides on the road, in breach of EU Regulation 715/2007, and they bring claims in contract, under consumer statute and in tort.

The court chose five lead defendants, Mercedes-Benz, Ford, Peugeot-Citroen (part of Stellantis), Renault and Nissan, to run a legal-issues and prohibited defeat device (PDD) trial addressing what counts as a defeat device under the Regulation and whether particular devices in test vehicles breached it. That trial ran from October 2025 to March 2026 before Cockerill LJ, sitting as a High Court judge, with dozens of barristers from chambers including Brick Court, Fountain Court, One Essex Court, Henderson Chambers, Crown Office Chambers, 2 Temple Gardens, Maitland Chambers and Monckton Chambers instructed across the many defendant groups.

On 10 July 2026, Cockerill LJ handed down a 369-page judgment. She adopted a narrow definition of defeat device, finding that only devices which detect a regulatory test cycle and function differently, and more favorably to emissions, only when a test is detected will qualify. Applying that test, she found only one of ten alleged Mercedes devices and one of six alleged Peugeot-Citroen devices to be prohibited defeat devices, and dismissed all allegations against Ford, Renault and Nissan. Both claimants and Mercedes have said they are considering appeals.

Following judgment, claimants representing roughly 450,000 Mercedes owners applied to stay the wider litigation and vacate the quantum trial listed for October 2026, arguing the lead defendants’ near-total success on liability undermined the case for proceeding immediately to damages. The court refused that application, meaning a quantum trial addressing remedies for the vehicles found to carry prohibited devices remains scheduled, while permission-to-appeal applications against the liability findings are expected to proceed in parallel.

TIMELINE

OCT 2026HEARING

Quantum trial scheduled

A further trial is listed to determine the consequences of the actionable breaches found in the July 2026 judgment, including damages and other remedies for affected Mercedes and Peugeot-Citroen vehicles.

High Court finds largely for manufacturers in 'behemoth' diesel emissions litigation
AUG 2026PROCEDURAL

Court refuses claimants’ application to stay proceedings and vacate quantum trial

Around 450,000 Mercedes claimants sought to stay the wider litigation and vacate the October 2026 quantum trial pending a possible appeal of the liability judgment. The court refused the application, so the quantum trial remains listed.

Court refuses claimants' application to stay the NOx emissions litigation against Mercedes
JUL 10, 2026APPEAL

Both sides signal possible appeals

Leigh Day, acting for around 1.6 million claimants, and Mercedes-Benz both indicate they are considering seeking permission to appeal aspects of the liability judgment.

Lawyers considering appeal following Dieselgate judgment Mercedes-Benz and Leigh Day Consider Appeals After High Court Dieselgate Ruling
JUL 10, 2026JUDGMENT

Cockerill LJ hands down PDD liability judgment

The High Court rules that only one of ten alleged Mercedes devices and one of six alleged Peugeot-Citroen devices are prohibited defeat devices, and dismisses all allegations against Ford, Renault and Nissan, in a 369-page judgment setting the English law test for a defeat device.

Judgment handed down in Diesel Emissions litigation: Court rejects large majority of allegations against car manufacturers Judgment handed down in the Pan-NOx Emissions litigation Carmakers Broadly Win Round One in Massive UK Suits Over Diesel Emissions
MAR 2026HEARING

PDD trial concludes after extended hearing

The lengthy trial on the correct legal test for a prohibited defeat device, and its application to the lead defendants’ vehicles, concludes after running since October 2025.

Judgment handed down in Diesel Emissions litigation: Court rejects large majority of allegations against car manufacturers Diesel defeat device trial enters final phase as DfT probes 110 models
OCT 2025HEARING

Prohibited defeat device (PDD) trial begins

The legal-issues and PDD trial against five lead manufacturers, Mercedes-Benz, Ford, Peugeot-Citroen, Renault and Nissan, opens before Cockerill LJ in the High Court.

Public do not trust car manufacturers as new Dieselgate case begins in High Court

EXPERIMENTAL AI FORECAST

HOW IT IS MADE →

Predicted outcome: Respondent win
Model-assigned probability: about 55%
Timeframe: late 2027 to 2029
Model confidence: medium

The most likely end state, at roughly 55 percent, is that the manufacturers substantially prevail, with Cockerill LJ’s narrow defeat device test surviving appeal and only a small residue of Mercedes and Peugeot-Citroen vehicles carrying any liability, resolved by the quantum trial or a modest follow-on settlement by late 2027 to 2029. The July 2026 judgment is the dominant driver: three of five lead defendants were cleared outright and only two of sixteen challenged devices were found prohibited, which collapses the settlement value of a book that was pleaded in the billions. The court’s refusal in August 2026 to vacate the October 2026 quantum trial keeps momentum with the defendants and forces claimants to prove loss on a shrunken factual base, where realistic per-vehicle awards look closer to the 2022 VW group settlement level of about £2,000 than to the headline demand, implying total residual exposure in the low hundreds of millions rather than multi-billion. Funding economics reinforce this, since adverse costs across fourteen defendant groups and the strained finances of the claimant firms make continued all-out prosecution of dismissed cohorts hard to sustain. The leading alternative, at roughly 25 to 30 percent, is a materially mixed result in which the Court of Appeal, pressed by the tension between the narrow cycle-detection test and the broad wording of Article 3(10) plus post-2020 CJEU thermal-window authority, widens the definition and revives large parts of the book for a further liability phase or a renewed multi-hundred-million settlement. A clean claimant victory restoring the full 1.6 million claimant case is unlikely, and outright abandonment of the appeal is possible but less probable than a fought permission application. Expect the quantum ruling first, in the first half of 2027, with the appellate answer, and possibly a Supreme Court petition on the defeat device test, running into 2028 or 2029.

July 2026 liability judgment cleared three of five lead defendants · Only two of sixteen alleged devices found prohibited · Court refused stay, quantum trial proceeding on narrow base · Claimant funding strain and adverse costs across fourteen defendants · Appeal risk from CJEU divergence on defeat device definition

Generated automatically by claude-opus-5 on Sep 20, 2026 from this case’s record on this site, and published without editorial review. The probability is the model’s judgment, not a measured frequency; there is no established validation record for these forecasts. Not legal or investment advice. How the forecasts are made →