Tory frontbencher Lord Bowness, a former Croydon LBC leader, argued during the committee stage of the Greater London Authority (Referendum) Bill that the poll should be deferred until more details were known about the proposals.
The Tory amendment to delay it until at least eight weeks after the detailed legislation is published was passed by 128 votes to 122 - the first government defeat of 1998. It coincided with the Cabinet committee discussing reform of the House of Lords.
Junior environment minister Baroness Hayman warned that the cost to councils of holding the referendum on a different day from the London borough elections could be about£3 million.
'We fully intend to go back to the Commons to restore the right of Londoners to vote in a referendum in May this year.'