A battle to run Brighton and Hove City Council has left the current Tory leadership effectively powerless.

Labour, Green and Liberal Democrat councillors will today unite to out-vote the Conservatives on one of the most important documents outlining how the city is run.

The three groups will vote together on 91 joint amendments to the city's 10-year masterplan, transforming it from the Tory vision for Brighton and Hove into their own.

It will be the first time the three parties have united since a by-election win for the Greens in July gave the opposition more combined votes than the Conservatives, by 28 to 26.

They will use the amendments to make key policy changes, including encouraging sustainable transport ahead of car use, changing housing allocations and transforming Valley Gardens in central Brighton into one large park.

The amendments were presented to Conservative council leader Mary Mears yesterday but last night she said it was too early for her to comment.

The leaders of the three opposition parties, who last summer rejected the possibility of a formal coalition, have now spoken of their willingness to unite again on key issues, including the council's budget.

The move signals the start of what may be a problematic two years for the Tories, whose vote share prior to the by-election had enabled them to appoint the council's leader, cabinet and mayor.

They will now face difficulty passing any of their key policies until the next council elections in 2011.

The opposition group leaders yesterday said they had been left with little alternative.

Labour group leader Gill Mitchell said: "Before the Conservatives came to power there was an all-party working group for the city's masterplan, the core strategy, but they scrapped it. They decided to work alone. The other parties have all been willing to talk to each other, to consider options and compromise and have come up with a shared set of amendments.

"It should not have needed to come to this and would not have if the Tories had been willing to work with us."

Green leader Bill Randall added: "We believe these amendments better reflect what people in Brighton and Hove actually want."

Lib Dem leader Paul Elgood said: "The Conservative core strategy doesn't represent the city, it could be for any city in the country. We've tried to put back in issues we feel residents feel strongly about."

Work on the core strategy started in 2002. It will set out how the the council wants the city to change up until 2026 and includes all aspects, from the economy to building development and transport. It is due to be finalised next year.

Of the 91 amendments planned by the opposition parties, 39 will be around transport.

The party leaders said they wanted to reverse the Tories' "pro-car" policies in order to reduce pollution and congestion in the city.

They have proposed reinstating conditions on some new housing developments which ban residents from owning cars, investing in new cycle lanes and pedestrian networks and developing a high speed public transport system along the coast.

Conservative leader Mary Mears last night said: "The amendments were published late today and I only received them shortly before this afternoon's cabinet meeting. I haven't had a chance to consider them yet so I can't make a comment.

"We have always stood by our Conservative views. We represent the majority in the city and they have made clear what they want."