Stop your crowing about Cameron leaving Britain marginalised, lefties. The proposed EU treaty [2] is perhaps the biggest catastrophe to befall the European left since the Second World War.

Sounds like semi-deranged hyperbole? Consider this: as Paul Mason has written [3], "by enshrining in national and international law the need for balanced budgets and near-zero structural deficits, the eurozone has outlawed expansionary fiscal policy".

Read that last bit carefully. Left-wing governments of all hues will, in effect, be banned by this treaty. If the French or the German left returns to power in the near future (and both are in a good position to do so), it will be illegal for them to respond to the global economic catastrophe with anything but austerity. An economic stimulus is forbidden – because the treaty has buried Keynesianism.

Cameron opposed the treaty because he feared the effect it would have on the City, which, after all [4], bankrolls his party [5]. But just because he opposed the treaty doesn't mean the automatic response of the left should be to throw its weight behind it. I proudly marched against the invasion of Iraq; I wasn't deterred by the fact the BNP opposed it, too.

François Hollande – the Socialist candidate [6] for the French presidency – has already spoken out against a treaty cooked up by Europe's overwhelmingly right-of-centre governments. If we're going to listen to European leaders, Hollande is a sounder bet than avowed right-wingers like Nicolas Sarkozy and Angela Merkel.

After this stitch-up, the left really needs to have a long, hard think about its attitude to the EU as it is currently constructed. There's still a sense that any criticism of the EU puts you in the same box as swivel-eyed Ukip-ers who rant about gypsies in shire inns. But there's a powerful left critique that needs to be made.

We've already had elected governments in Italy and Greece toppled by the bond markets with the complicity of senior EU figures. Successive compacts (such as the Lisbon Treaty) have enshrined the privatisation of public services. It was EU Directive 9/440 that made it a legal requirement for private companies to be able to run train services, and the European Court of Justice has issued judgments that have attacked workers' rights, much as making it possible for individuals to sue unions.

The new treaty is just the latest attack on European democracy – and against the European left. So let's stop taunting Cameron, and start working out how we can unite with the European labour movement to stop this total disaster in its tracks.

Owen Jones's "Chavs: the Demonization of the Working Class" is published by Verso (£14.99).