Peers have handed the coalition government another defeat, over plans to introduce five-year parliaments.
An amendment moved by David Pannick requiring the Commons to approve the five-year period at the start of each parliament was passed by 190 votes to 184.
That means ministers will have to persuade MPs to override the Lords when the fixed term parliaments bill returns to the Commons if they want to ensure the change becomes permanent.
Until now prime ministers have been able to choose when to hold a general election at a time which is most politically advantageous to them - with a maximum period of five years.
The Conservatives and Liberal Democrats have agreed to bind themselves to a full five-year term and are legislating to ensure this continues permanently.
Lord Pannick said there was "deep unease on all sides" of the Lords about the coalition's proposals.
He said the "sunrise provision" would help limit "what many noble Lords regard as the constitutional damage which would be caused by this unhappy bill".
"I suggest that we should accept... the political reality that the coalition wants a binding commitment for this parliament, but that we should stand firm in our belief-held on all sides of the House - that the case for general constitutional reform simply is not made," Lord Pannick told peers.
Betty Boothroyd, the former Speaker of the Commons, said she supported the amendment.
"I understand perfectly the coalition's wish to serve for a fixed period of years, to tackle the current economic situation and to see that its programme is enacted," she explained.
"However, I reject the same imposition being placed on the freedom of action of future parliaments."
Liberal Democrat peer Paul Tyler pointed out that early elections were still possible under the government's proposals - through a two-thirds majority in the Commons or a no-confidence vote which was not followed by a new government within two weeks.
"We have already downgraded the fix to something no more adhesive than Sellotape," he said.
"The amendments take us even further down the scale. They would turn the bill into the Blu-Tack bill or the Post-it note bill and would not be a fix at all."
The government is expected to restore the bill's original proposals, removing the Lords amendment, when the legislation returns to the Commons.