Green candidate for London mayor Sian Berry has urged her supporters to give their second preference votes to rival Ken Livingstone.

The move - matched by a promise from Mr Livingstone that he will give his own second preference to Ms Berry - came as the pair presented a united front through a joint green agenda for the next four years.

In a joint statement, the two candidates warned that Conservative rival Boris Johnson "cannot be trusted with London's environment".

The pair set out five green commitments on Wednesday, with both claiming it was vital that London maintained its strong green agenda.

Their green pledges include:

  • Introducing a £25 a day CO2 charge for gas-guzzling cars like some 4x4s, high-powered sports cars, and luxury executive vehicles
  • The size of the Congestion Charge Zone will not be reduced.
  • Low Emission Zone will be maintained and also extended to include lorries over 3.5 tonnes and, from 2010, heavier vans.
  • Oppose all further airport expansion serving London, including at Heathrow.
  • A target that at least 50 per cent of all new houses should be affordable to Londoners on lower incomes.

Livingstone, who is seeking a third term as mayor, maintained London's communities and neighbourhoods would all benefit from looking after the environment.

Meanwhile, Boris Johnson - 12 points ahead of Livingstone in the latest opinion poll - issued a warning to London's boroughs that he will "name and shame" any which fail to use powers to crack down on binge drinking and alcohol-related violence.

Voters can cast two votes for Mayor on May 1 - a first and a second preference. After all the first preference votes have been counted, the two candidates with the highest totals go into a final ballot.

The second preference votes cast by people who supported lower placed candidates are then counted and added to the first preference totals of the two top placed candidates.