Europe: Judging the rebellion

David Cameron cannot lose today's vote so the question that matters is how many Conservative MPs refuse to follow his lead.

Downing Street is briefing that the number it expects is 85 which - given the need to massage expectations - suggests that they think it will be significantly lower than that.

If the number of Tory MPs not voting with the government is...

42+

• This is the largest Conservative rebellion so far in this Parliament.

• This is also the biggest ever rebellion on Europe for the Conservatives when in government - bigger than the 41 Conservative MPs who voted against the third reading of the Maastricht Bill on 20 May 1993. Remember, however, that 41 votes against a government fighting for its life is much more significant than 42 votes for a backbench motion with no chance of becoming law. (Read more on this on Professor Phillip Cowley's excellent blog.)

• A large enough rebellion to defeat the government if, on another occasion, the rebels could unite with all MPs from all the opposition parties. (The government's formal majority is 79. However, because the Sinn Fein MPs don't take their seats, the working majority is 83. That means it would take a rebellion of 42 Conservative and/or Lib Dem MPs to defeat the government outright.)

42-61

• Team Cameron will point out that fewer Tory MPs will have rebelled than signed the motion which called for an EU referendum even though this will still be the largest Conservative rebellion so far in this Parliament and the biggest rebellion on Europe ever when the Conservatives have been in government.

61+

• All David Cameron's, William Hague's and the Tory whips' efforts have come to naught. There are more rebels than signed the original motion.

96+

• More than half the Tory backbenchers have defied an order to vote with the government - and it's the biggest rebellion by Conservative MPs in the post-war era. Dynamite.