Mayor of London Boris Johnson will today attend a cross-party rally against the highly controversial proposal to build a third runway at Heathrow airport.
Organisers say they expect thousands to turn out for the protest in Barnes, west London, to voice their opposition to expansion of the travel hub.
The Mayor has previously accused David Cameron of kicking the matter "into the long grass" by delaying an independent report on the future of aviation until after the next election.
He favours a new "Boris island" airport in the Thames Estuary or possible expansion at Stansted.
Former transport secretary Justine Greening is expected to make her first speech on Heathrow since she was moved to international development in September's Cabinet reshuffle amid speculation that it was due to her stance on the issue.
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David Cameron said the Met proved it can "do more with less" and declined to criticise the Mayor of London for his water cannon spend.
A group of disabled activists threw toys and balls at him as he entered the conference.
A wet start to the week and the unsettled weather is set to continue over the next few days.