Sadiq Khan today issued a joint appeal with the mayor of Paris for EU chiefs not to water down toxic air limits and risk killing 16,000 more people a year.
I've been working tirelessly on this very un-sexy sounding but crucial piece of air quality legislation, which is currently being hammered out by EU national governments and MEPs in the European Parliament.
The message from Europe's two biggest cities is now crystal clear. London and Paris have set out ambitious measures to tackle air pollution, and they do not want these efforts to be fatally undermined by UK government inaction at a European level.
The new rules are a once in generation opportunity to tackle the air pollution crisis blighting Europe, the UK government must stop its efforts to undermine them, learn from the mistakes of the past, and take the necessary action to tackle London's air pollution crisis.