The move follows an ultimatum issued a fortnight ago by the European Commission urging the Department of Transport to justify destroying 8,000-year-old Oxleas Wood, in Eltham, south-east London, or face legal action.
Campaigners opposed to the scheme announced the formation of the alliance in central London. To celebrate, a giant EC flag was also raised at Severndroog castle, in the woods. Dr Barry Gray, chairman of the protest group People Against the River Crossing, said: 'We have gone to great lengths to make the Government see reason and halt this road scheme.
'The commission's action has vindicated PARC's complaint that the UK Government has not carried out a proper environmental assessment of this scheme and is treating the environment with contempt.
'If the Department of Transport attempts to begin construction, then we have no choice but to take direct action in order to protect the local communities and their environment and to uphold European law.'
Robin Maynard, countryside campaigner for Friends of the Earth, said: 'FoE has been campaigning on Oxleas Wood for eight years. In that time over 1,000 key wildlife sites across the UK have been damaged.
'This destruction has to stop. For Oxleas Wood the next 12 months are crunch-time. The Government's fading green credentials hang on it stopping its road-mad transport department from carving up this 8,000-year-old woodland. If it won't, we will.'
The other groups in the alliance are Earth First, the World Wildlife Fund UK, the London Wildlife Trust, Alarm UK and the London Cycling Campaign.
The row over Oxleas - a Site of Special Scientific Interest - resulted in High Court action this year. The case went against the protesters and is due to go to the Court of Appeal. They have vowed to go to the House of Lords if necessary.
The Government says it has not broken EC rules on the environment and all the correct procedures have been followed.