The battle of Thermopylae is famous in legend for the sacrifice of 300 Spartans.  They died in battle, but saved their city.  The tale has a modern day Brexit resonance.

As we approach a third “meaningful” vote on Theresa May’s Brexit deal, the number of Conservative MPs still willing to oppose it is falling.  It was 118 in January, and 75 last week.

Switchers for last Tuesday’s second vote included David Davis, Graham Brady, Philip Davies and our columnist, Robert Halfon.  Among those who now suggest that they will switch on a third are Esther McVey, Simon Clarke and Daniel Kawczynski.

Which returns us to Thermopylae.

ConservativeHome is told that a hardcore of those determined to hold out now refer to themselves as “the Spartans”.  These include a significant chunk of the ERG – though calculations are complicated by the fact that not all those who oppose the deal are ERG members.

If the Prime Minister’s deal gets through, among the corpses of MPs slain in the pass should be those of: Peter Bone, Bill Cash, Christoper Chope, Mark Francois, Andrea Jenkyns, John Redwood and, we believe, Steve Baker.

Others who died at Thermopylae include Thespians, Helots and Thebans, history tells us.

Readers must decide for themselves which of these labels best describe Dominic Grieve’s band of pro-Second Referendum holdouts, but they, too, will surely stick against May’s deal – a fact that many of our media colleagues tend to overlook.

Last week, they included Guto Bebb, Damian Collins, Justine Greening, Sam Gyimah, Jo Johnson, and Grieve himself.  It is unlikely that many of them will peel off.

As we write, Downing Street is striving to win the DUP over to the deal.

If it succeeds, the calculation for May will be whether enough Opposition MPs will back her deal to cancel out the Spartans who oppose it.  We would say that the former are among the Persians, but are in danger of stretching this historical analogy way too far.

Among those well placed to pronounce on the question is Boris Johnson.  What will he do when the vote comes?  Will he stand with the Spartans, and return “with my shield or on it”, as he sometimes likes to write?  Or will he swap sides and join the Persians?

His Daily Telegraph column today is ambiguous on the point.  We are less qualified to pronounce on classical history than the former Foreign Secretary, but can’t help questioning whether the analogy holds at all.

For in this case, the city wouldn’t be saved if the Spartans are massacred, since a consequence of their defeat would be the deal passing.

And in any case, this time round, the Spartans may actually win.