Questions over Garden Bridge process
Transport for London officials met the designer of the planned Garden Bridge four times before the procurement process began in February 2013, documents seen by the BBC reveal.
These meetings happened before other firms were invited to tender.
In the documents there was no reference to a garden bridge - just a request for a design of a pedestrian footbridge.
A spokesman for the Mayor of London said the procurement process had already been found to be fair.
The documents - obtained through a Freedom of Information request by the Architects Journal - show designer Thomas Heatherwick first met officials on 24 September 2012, with London Mayor Boris Johnson in attendance.
At the next meeting on 17 December 2012, Mr Heatherwick met a deputy mayor to discuss what was described as the "next steps" for a Garden Bridge.
There was then a planning proposal meeting on 31 January 2013 and a final meeting - with Mr Johnson in attendance - on 1 February 2013.
Will Hurst, managing editor of the Architects Journal said he thought "the process smells very bad".
He said: "What alarmed me was that Wilkinson Eyre and Marks Barfield [the other two firms invited to take part in the procurement process] were outscored by Heatherwick on bridge design experience.
"I thought how on Earth can that be fair given that Heatherwick had only designed one tiny bridge in Paddington. The other two firms had designed many, many bridges."
There is no suggestion of any wrongdoing on the part of Mr Heatherwick.
A spokesman for the Mayor of London said: "An audit of Transport for London's procurement process has already found that it was open, fair and transparent...
"Work on building the bridge is due to begin this year and is widely supported by Londoners."
The Garden Bridge was conceived by the actress Joanna Lumley as a memorial to Princess Diana.
She is an associate of the studio of Mr Heatherwick, who also designed the Olympic cauldron and London's new Routemaster bus. He helped develop the idea before she lobbied Mr Johnson.
In January 2016, The Architects' Journal reported that, before the procurement process began, Mr Heatherwick was in San Francisco and joined Mr Johnson for a meeting with technology firm Apple to ask them to sponsor the bridge.
The mayor said that meeting had no bearing on a fair procurement process.
Prof Christopher Bovis, an expert on procurement who has advised the UK government and the European Commission on fair process compliance, said: "The procurement process of the Garden Bridge is littered with procedural irregularities. It is not one of the best examples of a procurement process in the UK."
The three firms involved have declined to comment but the head of the Royal Institute of British Architects, Jane Duncan, has previously called for the Garden Bridge project to be halted and scrutinised before more public money is spent.
The Department for Transport and Transport for London have both invested £30 million in the project.
The Garden Bridge Trust said it had now raised £85 million in private money for the bridge and would repay £20 million of the TFL money over the next 50 years.