David Cameron better have some pretty impressive green policy announcements up his sleeve for his set piece address at next week's Conservative Party conference. Otherwise the Tories' annual jamboree in Manchester will go down in history as the conference where the party ruthlessly trashed the environmental credentials it had spent five years painstakingly amassing.

That may sound like hyperbole, but it is hard to overstate the symbolic significance of the devastating one-two the Conservatives have delivered to the low-carbon economy over the past 24 hours through Philip Hammond's plan to raise the motorway speed limit to 80mph and Eric Pickles' pledge to restore weekly bin collections. The environmental movement has been left reeling. "How could they do this?" they ask. "How could they pursue such overtly high-carbon policies and still claim to be the 'greenest government ever'?"

In many ways, the scale of the shock exceeds the actual impact of the two policy stunts.

Hammond's ploy to secure the Top Gear vote (even though the high priest of petrolheads Jeremy Clarkson is on record as opposing an increase) will lead to a rise in the number of accidents and a surge in carbon emissions of between five and 15 per cent, depending on who you listen to. However, if the government's other policies and targets designed to encourage people to drive less and use greener vehicles work, then the inevitable increase in emissions should be wiped out over time.

Similarly, Pickles' utterly fatuous push to drive a coach and horses through his supposed localism agenda and bribe councils into reinstating weekly bin collections will inevitably make it harder to maintain recent improvements in recycling rates, lead to increased waste levels and directly contribute to an increase in landfill emissions. But it remains to be seen how many councils will take up the new funding given estimates that the grants will not cover the full cost of switching back to weekly collections. Moreover, if they are clever, councils may get away with simply restoring weekly food collections at the same time as stepping up investment in anaerobic digestion facilities capable of turning that waste into energy.

These twin policies are environmentally damaging, will result in increased carbon emissions, and show a shocking disregard for cabinet colleagues at departments such as DECC and the department for health, which argued against them. But the real shock comes from the way in which they lay bare Conservative priorities.

David Cameron knew he had to throw the right of his party a couple of bones during conference week, but the way in which two anti-green policies have been served up reveals his true disregard for the environment. Equally, the way in which £250m has suddenly been found to tackle the apparently pressing social problem that is weekly bin collection again reveals how appeasing the Daily Mail remains a higher political priority than building a sustainable economy.

As with the decision to cut fuel duty, the additional funding for bin collection reminds people that even at a time of deep spending cuts, decisions are still made on spending priorities. Playing the fantasy budget game is a futile experience, but the £250m earmarked for bin collections could have guaranteed the UK the world's best marine energy industry, creating thousands of jobs, just as the money George Osborne dished out in fuel duty cuts earlier this year could have helped turn the Green Investment Bank into a genuine engine for green growth.

Most significant, however, is the undertow the anti-green Pickles-Hammond clique in the cabinet applies to the wider low-carbon economy agenda.

The government remains committed to driving through the big policy reforms, such as electricity market reforms, renewable energy subsidies (despite an unexpected setback for the Renewable Heat Incentive), the Green Deal and the Green Investment Bank, all of which will help to accelerate low-carbon investment. Meanwhile, the Climate Change Act and its binding and ambitious emissions targets provide a framework that ensures that when the government does pursue higher carbon policies it has to find cuts elsewhere to compensate. The direction of travel in favour of a low-carbon economy is clear, and businesses should invest and adapt accordingly.

But the willingness to allow certain departments to repeatedly pull in the opposite direction only serves to fuel investor uncertainty, making it ever harder for the likes of Chris Huhne and Greg Barker to deliver the low-carbon future the government has promised.

Just as the previous Labour government saw the green gloss knocked off its Climate Change Act by its insane decision to support a third runway at Heathrow, the coalition will find that just one or two carbon-intensive decisions have a symbolic and huge impact, even if you are making good progress in other areas. Every time a minister pushes for faster cars, or argues for the creation of a domestic shale gas industry while barely acknowledging the carbon implications, the "greenest government ever" tag looks ever more ironic.

David Cameron urgently needs to pull rank and force the anti-green forces back in to line, or risk losing the environmental credibility he has spent years trying to build. He apparently did precisely that to force through ambitious carbon targets for 2025, despite opposition from his chancellor and others within the cabinet. But he now needs to make his support visible for green policies that are being worked on right now, starting with the imminent decision on whether to impose mandatory carbon reporting requirements on large firms.

The prime minister must next week rectify his failure to make a single speech on the environment since taking office and map out a clear green vision. Either that, or he risks leaving the low-carbon economy stuck in the slow lane, while his emissions-loving colleagues and their high-carbon policies speed off over the horizon.