The mayor of London has confirmed he met Thomas Heatherwick before his firm was awarded a contract to design the Garden Bridge.
A spokesman for the mayor acknowledged the meeting, which took place in California in 2013, for the first time.
London Labour said it showed "utter contempt" for the rules preventing undue influence.
Mr Johnson's spokesman insisted it had "no bearing" on the "fair and transparent" procurement process.
Transport for London published an invitation to tender for the design of a bridge connecting Temple to South Bank in February 2013.
The same month, the mayor travelled to California for a meeting with Apple. Thomas Heatherwick was there at the same time for a separate discussion with Apple.
The mayor's spokesman said that, "given that he had already expressed interest in creating a garden bridge, the mayor invited him to join the meeting and outline his ideas".
'Conflict of interest'
In March 2013, the contract was awarded to Heatherwick Studio and Arup engineers.
Labour's leader on the London Assembly, Len Duvall, said: "What you've got is the Mayor clearly supporting the Heatherwick bid, indeed actually fundraising for it, before the procurement process had even been announced."
He claimed it showed a "deep conflict of interest" and called for "full disclosure".
The Lib Dems and Greens have also challenged the mayor to provide more information about what happened at the meeting.
Mr Johnson's spokesman responded: "That meeting had no bearing on the procurement process led by Transport for London for the design of the Garden Bridge, which was open, fair and transparent."