The Lords is using an obscure tactic to try and force through votes for 16-year-olds

The House of Lords is going to continue with its attempt to give 16 and 17-year-olds a vote in the European referendum on Monday by voting on whether to add another amendment to the EU referendum bill. If the amendment passes, it will kick off something called parliamentary ping-pong and confirm that the Lords is willing to use an unusual legislative tactic to force its agenda through.

This fight between the House of Lords and the House of Commons is happening because the two Houses are completely split on the issue of giving votes to 16 and 17-year-olds. The Conservative-led House of Commons is opposed to the idea, while the majority of the House of Lords are in favour.

The House of Lords can't propose new bills, but it can add any amendment it likes to a bill the House of Commons is trying to pass. The bill is then returned to House of Commons to see if the Commons is happy with the amendment.

Most of the time this is a pretty straightforward process. The House of Lords scrutinises bills sent to it by the Commons, adds some amendments and sends the bill back to Commons. If the Commons doesn't quite agree with the amendment, the bill goes back and forwards between the two chambers a few times until they agree on the wording and the bill can then become law.

However, when the EU referendum bill was sent to the Lords, they didn't just change the wording a bit. They added an amendment that would change the whole voting franchise as the UK's voting age is 18.

The House of Commons was so angry at the Lords, that they didn't just reject the amendment, they got the Speaker of the House of Commons to rule that lowering the voting age would cost £6 million and was therefore a "financial" measure. This was a clever thing to do because there is a long-standing convention that the House of Lords doesn't interfere in financial bills.

That brings us to where we are now. Instead of backing down, the new amendment the Lords is tabling basically says that it won't cost that much to register young people to vote and suggests "low-cost means of communication such as email" to help get people registered to vote. By doing this, the Lords is saying it is happy to keep sending new amendments back to the House of Commons until it gets its way.

In theory, the bill could keep been passed back and forwards like the for a whole session of parliament without an agreement being reached.

There is one final twist though - both Houses have a nuclear option. The House of Commons can use something called the Parliament Act to force the bill through, though that can be a time consuming option, while the House of Lords can pass a "motion to adjourn consideration," which basically means it is going to refuse to make any more amendments and the bill will be lost.

If the amendment passes in the Lords today, the two Houses will in effect be locked in a high-stakes standoff. The government really needs to get the bill passed before Christmas to ensure the EU referendum can be held in 2016, while the Lords has time on its side and seems happy to keep the political ping pong going for as long as possible.