Royal Parks chief executive Mark Camley advised that although the union would become law in November 2011, it might not take effect until after the London Olympic Games in 2012.
After last week's 25 per cent cut in Government funding, he added: "One of the things the Mayor is keen on is philanthropy and sponsorship - for example, the bikes scheme and what he's done in terms of the Olympic Park.
"We, along with the Royal Parks Foundation, hope the mayor could bring the same persuasive powers to potential sponsors and donors to bring funding to the park."
Camley said sponsorship, such as $1.25m (£790,000) from Tiffany & Co for fountain restoration, helps. Royal Parks also rents out areas for film-making such as the new Pirates of the Caribbean film.
But Brian Donohue MP, the secretary of the All-Party Parliamentary Gardening & Horticulture Group, said: "These parks have enormous benefits and I think they will suffer under the partnership. It does worry me and it worries me on the basis of experience."
KENSINGTON PALACE PREVIEW
Members of the All-Party Parliamentary Gardening & Horticulture Group were treated to a sneak preview of the £12m Kensington Palace restoration project last week.
Kensington Palace director Nigel Arch showed the group the plans, which he described as a "new open space for London" connecting the palace and Kensington Gardens.
The project is set for completion in March 2012 and aims to increase visitor numbers by 40 per cent, with a "kids go free" attraction including new education and community facilities.
Royal Parks head of parks services Dennis Clarke said its work ties in with that of Historic Royal Palaces and they were working to create a "21st-century interpretation" of the original garden plan.
He added: "We are working on detail design in terms of getting the right balance for people to get the very most out of their visit."