A Chinese developer has been given an ultimatum by a local authority to clarify its intentions for a "new" Crystal Palace or risk losing rights to the project.
Bromley Council has asked the ZhongRong Group for plans within 14 days.
The group wants to build a new cultural attraction and restore the surrounding park in Bromley.
Council leader Stephen Carr said it needed "to reassure local people" Crystal Palace Park would be improved.
The company's 16-month "exclusivity agreement" expired at the end of January. Bromley Council said it would start opening negotiations with other parties interested in developing Crystal Palace if detailed plans are not obtained.
The replica palace was announced with some fanfare by Mayor Boris Johnson in October 2013.
BBC London has learned Bromley is demanding a non-refundable £5m deposit as a condition for extending the agreement for a further six months and has set a deadline for a response by 20 February.
The original was built in 1851 in Hyde Park but moved to Crystal Palace in 1854. It burnt down in 1936.
Hotel, conference and entertainment facilities were also promised by the developer.
In October 2013, ZhongRong Group chairman Ni Zhaoxing said: "London is renowned across the world for its history and culture and the former Crystal Palace is celebrated in China as a magnificent achievement.
"This project is a once-in-a-lifetime opportunity to bring its spirit back to life by recreating the Crystal Palace and restoring the park to its former glory."
The company first started discussions with the mayor about the project in 2012.
In a letter, Bromley Council has asked the ZhongRong Group to provide details, including confirmation of the identity of its lawyers and other professional advisers involved in the scheme.
The council told the developer the non-refundable deposit would be used to cover the council's costs and for future improvements in the park.
The authority said it was unclear how the company would propose to help fund and manage the park in the long-term.
Council leader Stephen Carr said: "We need to reassure local people that we remain resolutely focussed on doing everything we can to improve Crystal Palace Park and this must include securing its long term future.
"It would also be wrong to overlook the exciting potential for much needed regeneration in this part of London."
Local campaigners and political opponents have been critical of how little has been revealed so far about the proposals, and the council's willingness to negotiate with one developer.
Tom Chance, Green Party parliamentary candidate for Lewisham and Penge said: "We should get rid of these developers, and commit not to going into any more secretive deals that override the planning masterplan and local wishes."