MPs are voting on changes to constituency boundaries that could see the Conservatives make large gains in the next election.
Coalition partners David Cameron and Nick Clegg are expected to enter opposing division lobbies today as the Liberal Democrats oppose Conservative plans to redraw parliamentary boundaries ahead of the next general election.
The Prime Minister wants to overturn a Lords defeat that means the boundary review will not be implemented until after the 2015 election. The new boundary could potentially hand the Tories up to 20 extra seats.
Nick Clegg announced last year that his party would attempt to delay the review after their coalition partners forced the abandonment of plans to reform the House of Lords.
However, while the Tories remain short of a Commons majority, Downing Street has acknowledged that the numbers for the vote look "pretty difficult".
Lib Dem peers joined Labour to back an amendment delaying plans for a boundary review and to cut the number of MPs until after the next general election, earlier this month.
The revolt was bolstered by six of the seven Lib Dem frontbenchers in the upper chamber, marking the first time in this Parliament that ministers have voted against the Government.
A spokesman for the Prime Minister yesterday said: "the application of collective responsibility has been set aside" for the Electoral Registration and Administration Bill.
Mr Cameron is not confident of overturning Lords amendments, his spokesman explained: "From the Prime Minister's perspective, the arithmetic looks pretty difficult."
"The Prime Minister's view is that it is very important that this issue comes before the House of Commons. The Prime Minister's view is that he is strongly in favour of smaller, cheaper and fairer politics but it is important that MPs can consider the issue."
The spokesman suggested no talks are under way with smaller parties in the hope of securing enough votes to force the Bill through.