One area where the troubled Coalition might have been expected to show a radical and unified sense of purpose is in relation to environmental policies. Before the formation of the Coalition, the Liberal Democrats had taken a consistently radical stance on most so-called green issues, a theme that united the party's disparate followers. David Cameron defined his leadership in opposition by a new focus on the environment. "Vote Blue. Go Green" was the party's slogan at local and European elections as part of the Conservative leader's claim to have modernised his party. With a flourish and "hug a husky" photocalls, Mr Cameron promised he would head the greenest government in history. On this issue at least, Conservatives and Liberal Democrats appeared to be at one.

The appearance was deceptive. As Prime Minister, Mr Cameron has not made a single speech on the environment. Far from bonding the two parties, environmental policy is fuelling fresh and potentially explosive tensions. So far the division has not been as noisily exposed as it was over reform of the House of Lords, but the split between the two parties is much starker and potentially even more serious. At least when Nick Clegg put forward his proposals for the House of Lords, the Liberal Democrats could count on the support of Conservative ministers. On the green agenda the division is between ministers at the top of the Coalition.

First, there is the persistent speculation that David Cameron and George Osborne support a third runway at Heathrow airport, having made much of their opposition on environmental grounds before the last election. More immediately, Mr Osborne is reported to be blocking new subsidies urgently required for renewable energy. The subsidies are being proposed by the Liberal Democrats' Energy Secretary, Ed Davey, and are entirely consistent with what both parties were arguing as part of their pre-election pitches.

Mr Osborne fears that what he now regards as over-generous support for wind power and other renewable sources will deter investment in gas-fired power stations, which he believes offer businesses and consumers the prospect of lower bills in future. But he and Mr Cameron made no such argument when they were claiming to be environmental crusaders. Not surprisingly, the Chancellor has the support of more than 100 Tory MPs who signed a letter this year calling for cuts in subsidies paid to promote the "inefficient and intermittent energy" supplied by wind farms. Most Conservative MPs were never swayed by their leadership's brief and cynical embrace of green issues.

The resistance from the Treasury exposes even more vividly than the paralysis over House of Lords reform the shallow pre-election claims from David Cameron and George Osborne that they had modernised the Conservative Party. Cynically the duo seem to have seized on the issue of the environment to prove that their party had changed. The new mood music was composed to appeal in particular to Liberal Democrat voters in key marginal seats and non‑Conservative newspapers. For a time the melodious tunes cast their spell, perhaps too well, as the duo's apparently new approach was never closely scrutinised.

Over the summer Mr Cameron must decide whether he wants to be seen as a modernising leader of a coalition or a leader of a right-wing Conservative Party like his election-losing predecessors. He chose the environment as a symbol of modernisation. Let us accept his choice of policy area and conclude that, if he fails to deliver adequate subsidies for renewables and declares his support for a third runway, his modernising project has died before it came to life.