David Marks, of Marks Barfield, revealed during a briefing with councillors that private funding for the i360 tower had been pulled.
But he said he was confident the £38m project will still go ahead.
Brighton and Hove City Council said its £15m loan offer still stood. The scheme also has a £3m enterprise loan.
The investors pulled their funding in the summer of 2012 but the information has only just emerged.
Mr Marks, who also helped to design the London Eye, said the investors had been "very reluctant to pull out".
He added: "The reasons they said had much more to do with the market and investor sentiment in their funds than the project, which is resilient and financially viable."
He said he was currently in discussions with other investors and that he and his wife Julia had invested their own money into the project.
Mr Marks added: "We care deeply about this project.
"We believe it's iconic in scale and design and it'll raise the profile of the city. We think it'll do a lot of good things for the city, it'll create jobs, it'll bring in visitors, it'll help more people stay overnight and it'll be a fantastic thing for the city.
"We will do this, this will happen."
Chairman of the economic development and culture committee, councillor Geoffrey Bowden, said he had confidence in Mr Marks.
But Brighton developer Mike Holland, who has previously expressed a wish to buy the nearby ruined West Pier, said the project did not have a hope of being realised.
He said: "Let's be honest about it, Nelson has more chance of getting his eye back than David Marks does of finding backing for this scheme.
"Let's stop talking about it, let's all stop being silly and let's get on and do something else. Let's tidy the seafront up.
"We either look for somebody to come along and rebuild in the old style or new style, or whatever, or we just take the thing away and we just tidy the railings and let the traders get on."
The developers believe the tower will attract up to 670,000 visitors a year.
Investors have withdrawn £15m from a project to build a 600ft (183m) viewing platform on Brighton seafront, according to the scheme's architect.