Peers face all-night sittings as the Government struggles to avert a parliamentary crisis threatening to derail the voting-system referendum in May.
Delaying tactics being used by Labour have so bogged-down the legislation that some peers have accepted that there may not be time for the Lords to fully scrutinise the legislation.
Ministers concede that the Bill, setting up a May 5 vote on the Alternative Vote system, is in trouble.
It must receive Royal Assent by February 16, or the referendum will be delayed. Labour objects to the way that the referendum has been twinned in the same Bill with the Tory drive to cut the number of MPs by 50 to 600. Labour fears that it will suffer the most from the boundary changes.