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Today the EU referendum bill begins its consideration in the House of Lords.

For those who do not follow the ebb and flow of the Europe debate, this is the bill that would commit Britain to hold an in-out referendum on our membership of the European Union.

The bill is being piloted through Parliament by a Conservative backbencher because the Liberal Democrats would not allow it to be government business.

The Tories hope the bill will convince voters they are serious about holding a referendum.

It has passed successfully - if rather tortuously - through the House of Commons and now it is the turn of peers to cast their eyes over the detail.

The overwhelming assumption at Westminster is that the bill will probably not make it out of the Lords alive. The Lords' constitutional committee warned as much last week.

None of this is definitive; the bill could survive. But it is very hard for private members' legislation to make it through the Lords without cross-party support.

And peers on all sides are lining up to bludgeon this measure to death by amendment.

So the question is what would happen then, later this year?

Most Tories expect - or at least hope - the government will use the Parliament Act to force the bill onto the statute book.

But I am told by senior government sources that this is by no means as automatic as some might think.

First, the Parliament Act is not something that the government can "use" or "impose". It would require a Conservative MP to come high in the ballot for private members' bills in the next session of parliament so they could reintroduce the original bill.

It is quite possible therefore that it would become very difficult to bring back the bill with enough time for it to pass through the Commons if Labour and Lib Dem bills took priority in the Parliamentary queue.

Second, for the Parliament Act to be used, the new bill would have to be introduced exactly as the original bill was when it left the Commons the first time around.

It would then have to pass through the Commons a second time without amendment for it to get onto the statute book without passing through the House of Lords. That could get quite tricky if time is short.

Third, the Liberal Democrats might not treat the second EU bill in such a benign way. First time around in the Commons, they have largely allowed the Tories to push the bill through without opposition, simply by refusing to engage with the issue.

The Lib Dems have let the Tories have their day in the sun without opposing hard. Closer to an election the Lib Dems might want to pick a fight. It is by no means certain that the EU bill would have a majority in the Commons if all sides chose to vote.

Fourth, the Conservative leadership itself may consider it a strategic mistake to allow the Parliament Act to be used.

David Cameron might not want to have a technical row in the Commons over referendum timings that close to the election.

It might look more like the Tories banging on about Europe rather than giving the people a say. He might consider his manifesto promise of a referendum by 2017 enough. And he might want to blame the Lib Dems for the failure of the bill.

Of course, the Tory leadership would come under huge pressure from its backbenches to use the Parliament Act in the wake of the EU elections in May and it might itself consider a legislative fight with the Lib Dems to be electorally advantageous.

But as of now, Tory MPs should not bank of the Parliament Act being used to force this bill through if it dies in the Lords.