Another £150 million is still needed for the redevelopment of New Street station, Birmingham's MPs and business leaders have been warned.
Detailed proposals have been drawn up to transform New Street from a "blot on a landscape" to a fitting gateway to a great city.
But only £200 million has been found to pay for the £350 million scheme, which civic leaders hope will be ready for the 2012 Olympics.
The warning was issued at a pioneering London summit yesterday, designed to improve the way the city gets its message across to decision-makers in Whitehall and Parliament.
The new campaign, dubbed Building a Better Birmingham, is supported by The Birmingham Post.
Representatives from the CBI, Institute of Directors, Birmingham Forward, Engineering Employers Federation and the Chamber of Commerce are to work with MPs and peers to present a united front.
City council leader Mike Whitby, a Conservative, spoke at the event, along with Lib Dem Paul Tilsley, the council's deputy leader, and Sir Albert Bore, leader of the Labour Group.
The meeting was held in Birmingham's prestigious London office, known as Birmingham W1, in Piccadilly.
Ambitious plans to redevelop New Street station have received support in principle from Advantage West Midlands, which has offered to contribute £100 million, and the Government Office of the West Midlands, which has indicated it could also provide £100 million.
But Mike Taylor, group leader for planning at Birmingham City Council, urged politicians to push for funding from other bodies such as the Department for Transport or Network Rail.
He said: "New Street now is a poor quality structure in the heart of the city. The image is particularly unpleasant."
The station had also been closed six times in the past six months because it was overcrowded, he said.
But he warned: "This is not a done deal, in respect of the business case. We need to keep the pressure on."