Why is Cameron ignoring the plastic bag tax open goal?

Is there a simpler environmental win available to the government than the introduction of a plastic bag tax?

The case is now so robust that it is hard to imagine why the Treasury remains so resistant to a proposal that commands significant support, in the cabinet, in the country, and in the media. And yet still proposals for a modest five pence per bag levy remain shunted into a siding, while the latest figures released this week confirm the supermarkets have once again stuck a metaphorical two fingers up at the Prime Minister, wilfully ignoring his explicit warning that he was fed up with their failure to reduce the number of bags distributed.

The latest data from WRAP shows the number of single use plastic bags distributed during 2011 rose 5.4 per cent to eight billion bags. This is bad enough, but it conceals an even worse picture for parts of the country. The Welsh Assembly's bag tax meant bag numbers there fell by 22 per cent, while the Scottish government's various waste reduction policies obviously helped keep levels of bag use flat north of the border. In marked contrast, bag use in England rose 7.5 per cent, while it climbed 8.1 per cent in Northern Ireland.

Now the Northern Ireland Assembly wants to do something about this woeful performance and will emulate Wales and the Republic of Ireland by bringing in a bag tax from next year, while the Scottish government last month launched a consultation on introducing its own levy. And from the Westminster government? Nothing.

According to the Westminster rumour mill, the Treasury put the kibosh on Defra plans for a tax earlier this year amid fears it would put more pressure on household finances. But with the latest figures categorically confirming retailers' voluntary efforts to curb the number of bags are failing – bag use has now been rising for around 18 months and (with a couple of honourable exceptions) there is still no sign of a renewed push from retailers to tackle the problem – the Treasury's position looks increasingly fool-hardy.

Let's take the most legitimate criticism of a bag tax first: it will put too much pressure on hard-pressed household budgets. I can understand why the Treasury does not want to be seen to imposing any further costs at this time. But this is not like a hike in income tax or VAT or even fuel duty (all of which they have at times in the past been happy to do in the name of the deficit). You do not need Jimmy Carr's acountants to avoid this tax. In fact it is the easiest tax in the world to avoid – just take a bag with you when you do the shopping.

Moreover, all the evidence from bag tax schemes around the world shows that they work. They are an example of an environmental tax at its most effective, in that they change behaviour and lead to less revenue being generated over time. The Welsh experience shows that the levy will not raise huge sums, and if the Treasury is worried about accusations of a stealth tax it can simply allow retailers to divert the money to the charity of their choice. The pressure on household budgets created by a bag tax will be negligible.

If the economic case for action is solid, the political case is hugely compelling.

The Daily Mail has campaigned for more robust action to tackle plastic bags and would almost certainly praise such a move as strong leadership from Cameron, while green groups can be relied on to welcome a move that would be seen as a symbolic victory. It would also allow Caroline Spelman to trumpet a significant environmental victory after the embarrassment of the forest sell-off u-turn and help the entire government demonstrate that it is committed to the coalition agreement pledge to raise the proportion of green taxes and serious about its "greenest government ever" sound bite.

The simple fact is that the retail industry's argument against the introduction of a bag tax rested on one pillar, and one pillar alone: that bag use could be curbed as effectively using voluntary measures. The argument was just about plausible when bag use was falling and supermarkets were making visible efforts to try and get people to take fewer bags, but it is about as much use as a soggy paper bag now that their performance is deteriorating again.

In any discussion of plastic bags it is always important to point out that this is a minor issue compared to the many greater environmental challenges we all face. But these bags remain a major source of litter and a powerful symbol of our unsustainable throw-away society. It is hugely frustrating that with a simple solution at hand progressive policies are again being blocked by the Treasury.

In failing to take action Cameron is left looking weak and once again in awe of powerful lobbyists. He told the supermarkets he would act if they did not improve efforts to tackle the problem and now it turns out plastic bag use is rising again.

On this issue supermarkets have had more final chances than John Terry. Ministers should warn them they have until the autumn to deliver deep cuts in bag use akin to that achieved in Wales, or else prepare themselves for an announcement in next March's budget confirming the introduction of a bag levy with immediate effect.