A representative said Johnson hit the brakes over worries about the knock-on effects of axing traffic from one of London's busiest and most integral nodal road points.
"He has asked for further work on the improvement project before giving the go-ahead for the next stages," she said of the £18m scheme by Vogt Landscape Architects. The Swiss firm had been commissioned by Livingstone early this year - the former mayor wanted a major tourist attraction by the 2012 London Olympics.
Vogt redesigned the area around the Tate Modern.But the plans hit controversy almost immediately when a removal of eight listed statues was suggested to free the square of "clutter".
The representative added: "The scheme has not been scrapped. We are following concerns on the possible traffic impact, which are supported by Transport for London.
"We are merely looking at the main concerns around traffic implications in conjunction with Transport for London.
"There are no timescales but it's being done urgently because we want Parliament Square in a good state for 2012."
Vogt Landscape Architects could not be contacted for comment.