On Saturday the Stirling Prize-winner was expected to herd a flock of 60 sheep across the Millennium Bridge - an event which the Biennale's organisers had hoped would bring in large crowds.
However Foster has now turned his back on the Biennale, claiming he is too busy in New York working on his World Trade Center proposals.
Instead, Pompidou Centre collaborators Richard Rogers and Renzo Piano have stepped in to take over the sheep drive which kicks off the capital's second architecture biennale.
Speaking about Foster's withdrawl, a spokeswoman for the Biennale said: 'It is not that Foster is being horrible, it's just that he is tied up in New York.'
The stand-in duo will herd the Herdwick sheep from Southwark Cathedral, south of the Thames, across the Millennium Bridge to Smithfield Market - echoing the historic drives to markets by local farmers dating back to the 12th century.
The event will highlight the massive changes which have taken place in the area and, in particular, the impact of the Millennium Bridge - the first new bridge across the Thames in more than 100 years.
Bennetts Associates, which masterminded the Longhorn cattle launch event at the inaugural 2004 Biennale, is also coordinating this year's sheep drive.