Ken Livingstone has accused Tory peers of denying Londoners a fair choice after they voted to limit the number of terms any mayor can serve to two.
Ken Livingstone has been elected twice as mayor
"It is up to Londoners, not parliamentarians, to decide who is mayor," said Mr Livingstone, who hopes to seek a third mandate in May 2008.
House of Lords Conservatives warned the office had become "the nearest thing we have to a dictator".
The government said it would look to overturn the move in the Commons.
The Lords voted 177 to 159 during the Greater London Authority Bill's report stage.
The Bill gives Mr Livingstone more powers over strategic issues such as planning and waste.
"This is further evidence that David Cameron has no confidence in his party's candidates for mayor," said Mr Livingstone.
"They seem to have decided they can't win next year's mayoral election through fair means so now they have resorted to changing the law to prevent a fair choice."
Opposition peers warned there was a risk that without the curb the office could become too powerful.
Baroness Hanham, a Conservative, told peers: "The office of mayor now in this country is the nearest thing we have to a dictator.
"There is very little that can stop the mayor doing what he wants to do."
She went on to deny that it had anything to do with the current mayor.
Mr Livingstone has twice been elected London mayor, once as an independent in 2000 and as the Labour candidate in 2004. The next poll is due in May 2008.
Opposing the move, Labour peer Baroness Andrews said: "To remove the right of Londoners to vote out the mayor is a fundamental change. It should not be contemplated lightly.
"If he is a dictator the best way to remove him is to remove him by a democratic process."