The Prime Minister has formally announced that the doctrine of Cabinet collective responsibility is to be suspended with regards to the vote on the boundary review. This grants the Liberal Democrats and Conservatives the right to whip against each other when the issue returns to the commons after the defeat in the Lords on Monday .
Collective responsibility allows for cabinet members to disagree in private provided they remain, in public, united. It's a crucial part of the so-called "payroll vote", the name for the core group of MPs who, by virtue having salaried government positions, will never rebel. In addition, it dampens down the damage of actual splits in opinion within cabinet.
That latter aim has been tested under the coalition for a while, with Vince Cable in particular being generally outspoken about his disagreements. But this marks the first time this cabinet will explicitly be allowed to split in a division in the commons. In fact, aside from a blip in 2003, when Clare Short was allowed to remain in the cabinet despite voting against war with Iraq – although she later resigned – it also marks the first time in the post-war era.
As George writes , lifting cabinet responsibility has happened several times when it comes to referenda. Wilson allowed his cabinet to campaign on opposite sides of the 1975 in/out referendum; and the coalition itself formally allowed a split over the AV referendum in 2010.
But as far as I can tell, this marks the first time collective responsibility will be formally lifted for a vote in the house since 1931, when Ramsay MacDonald's National Government was split over whether or not to introduce protectionist tariffs. It marks a large constitutional watershed – and raises questions about whether coalitions can ever be viable in the British political system.
Martin Shapland points out that the precendent is more recent than that:
@alexhern  Sorry mate. Frontbenchers voted both ways on the 1977 European Assembly Bill
— Martin Shapland (@MShapland) January 15, 2013