Boris Johnson has urged campaigners for Croydon Tramlink to be extended not to dismiss the possibility of improvements to other forms of transport instead.
The Mayor of London had previously given qualified support for proposals to extend Tramlink to Crystal Palace and Sutton and called on interested parties to submit business cases supporting improvements to the hugely-successful network.
But responding to question's at Mayor's Question Time last week, Mr Johnson refused to rule out underground extensions or railway improvements instead.
"You can deliver just as many benefits by properly integrating those Overground trains, increasing frequencies, improving the stations, for much less capital investment and much less disruption," said Mr Johnson, who said there was a "reasonable business case" for the Sutton extension.
"I also think it is important in south London not to shut our minds to the possibility of Underground extensions.
"One thing that worries me is that these are not popular in south London because people think 'Oh my god, if the tube comes, then we really are being colonised, we really are becoming part of London and we want to think of ourselves as leafy suburbs'.
"I am just putting it out there, as they say. It is important that we should not fetishise trams at the expense of underground railways."
Mr Johnson was responding to questioning from Steve O'Connell, the GLA member for Croydon and Sutton, who said business cases for tram extensions would shortly be presented to the mayor.
"One of the reasons why we are so keen to bring Tramlink to Croydon and Sutton is because up to now, when we have made representations around Underground, we have had an incredible pushback, not necessarily from yourself, but from others," said Mr O'Connell.
"It is refreshing to know that your door is open to this potential around tram, and I know my constituents would indeed welcome the Underground to come to their boroughs."
The question has to be asked would having a underground link to Croydon be a benefit for the area? with the increase expected of city workers and traffic congestion caused by the new Hammerfield& luxury property developments could this idea which Boris has hinted really work.