Joanna Lumley’s dream of a “floating paradise garden” to welcome weary commuters and tourists across the River Thames came one step closer to reality yesterday, as the actress and campaigner helped launch a trust to oversee the building of the country’s first garden bridge. The Garden Bridge Trust has been formed to organise the funding and construction of a bucolic river crossing in the heart of the capital – a concept that first came to Ms Lumley 15 years ago.

Speaking at the trust’s launch at the National Theatre, which overlooks the planned site of the crossing, the actress and Gurkha rights campaigner told The Independent that the bridge, which has been designed by London 2012 Olympic cauldron designer Thomas Heatherwick, will be “the tiara on the head of our fabulous city”. Yesterday it entered its first formal planning stage – public consultation – after an early £4m cash injection from Transport for London (TfL), but Ms Lumley said she hopes the flower-packed bridge will “be seen as a gift to the people of London from themselves for generations to come”.

The crossing, which organisers say could be completed by the end of 2017, is inspired by Ms Lumley’s memories of the mist-covered mountain garden at her childhood home in Malaysia.

Download the new Independent Premium app Sharing the full story, not just the headlines

Ms Lumley first joined forces with long-time friend Mr Heatherwick last year, just as TfL announced it was looking for proposal for a pedestrian bridge to cross the Thames from Temple tube station to the South Bank

“I want the bridge to be something people gawk at and wander across as the slowest way to cross the river,” said Ms Lumley. “It should be something that Londoners come together to give themselves and that will be tended long after we are all dead.”

Mr Heatherwick, who also created the award-winning “seed cathedral” at the World Expo 2010 in Shanghai and the new London Routemaster bus, said: “We’re proud of punching below our weight in this country, but the Olympics has shown that we can surprise ourselves with what we can achieve. We can dare to be ambitious.”

The garden bridge will be up to 30m across at its widest point. Its walkway will be up to 6m wide – around the same as the Millennium Bridge, which opened in 2000 and was christened the “Wobbly Bridge” after it swayed from side to side.

The Garden Bridge won’t face this problem though, as its two vast piers will act as “giant planters” and support 1,000 cubic metres of soil, plants and wild flowers, including alder and birch trees as well as a leafy glade of fruit and mulberry trees.

“Britain has phenomenal horticultural muscle,” said Mr Heatherwick. “We need to harness that along with public excitement to make this thing a reality – imagine if every Londoner gave £1, think how amazing it would be to walk over that bridge then.”

Lord Mervyn Davies, chairman of The Garden Bridge Trust, joked he still gets “stomach acid” when he thinks about the fundraising challenge ahead. But the former government minister and ex-chief executive of Standard Chartered said the challenges would be worth overcoming to show that London as a “truly every special city the rest of the world can continue to admire”.

The Garden Bridge Trust, which will submit formal planning permission next year, hopes to raise the majority of its funding from public donations,and both Ms Lumley and Mr Heatherwick say they do not favour large corporate donations that come with strings attached.

Mr Heatherwick said: “We have had some philanthropic families already offering us tens of millions of pounds and it has inspired me to realise we don’t want a big name on it. There have been other projects in London where a very big sponsor has come in and done an advert across it for themselves; if we possibly can, we’d like to avoid the bridge having a gigantic corporate name across it.”

He added: “This will only happened if British people decide to do something for themselves.” While Ms Lumley agreed that large-scale corporate sponsorship can have a “negative impact” and that it’s much “cooler” for large donors to be “sorts of kindly uncles would won’t care about having their name on it”.

Comparisons have been drawn with New York’s High Line, a mile-long park built on a former stretch of the city’s West Side train line. Mr Heatherwick says that, like the High Line, the Garden Bridge will have a series of “places to have trysts, plot, plan or propose”.

It’s unclear if there will be a place for cyclists on the new bridge, as designers are likely to suggest that they dismount.