Launching the Tories' energy policy in July 2006, David Cameron, gave a convincing and well-reasoned argument explaining why nuclear power must be a "last resort". Later that year he described Labour's enthusiasm for nuclear power as "irresponsible". As Cameron rightly pointed out: "The problems of nuclear waste haven't been dealt with. They have got to be dealt with in order to make any new investment possible."

Four years on, we're no closer to finding out how to deal with highly toxic nuclear waste and the Tory leader's point stands as strong as ever. But unfortunately, the Tories no longer seem to care. Indeed, the Tories' new green paper on energy security shows remarkable dexterity in rewriting history, now criticising the Labour government for dragging its feet before finally coming round to support new nuclear.

This political journey neatly epitomises the Conservative's new energy policy: confused, incoherent and lacking in credibility. Nuclear power has always required huge amounts of public money and David Cameron's signal that the Tories are ready to turn on the taps of taxpayer support risks billions which we simply can't afford. Both Labour and the Tories claim that they will not provide any public subsidy, but both know that this cannot be true when the nuclear industry that has never been able to survive without it.

Like the banks, new nuclear is too big to fail. And like the banks, new nuclear depends on a more or less explicit taxpayer guarantee. Once a nuclear power station is running we will have it for the next 40 years, come what may. No responsible government could ever let a nuclear power generator go bankrupt. This has happened before, when the taxpayer had to bail out British Energy £10bn and accept £73bn of their liabilities. As my colleague Vince Cable has said, nuclear power is the Royal Bank of Scotland of the energy industry.

There has been a lot of talk of energy security. It is the case that a lot of our generating capacity will come offline in the near, future leaving us with the possibility of an energy gap. If nothing is done now we will start to see an energy gap emerge around 2016. It is something that politicians from all sides must take seriously. It is also another reason why we cannot rely on nuclear power.

One nuclear reactor takes seven years to build. The designs for the reactors that have been proposed by the energy companies have yet to be approved, and the health and safety executive has said already that they will require a major redesign. Realistically we are at least a decade away from new nuclear power in the UK.

The only way we can guarantee a clean, safe and secure energy supply is to invest now in renewable energy which can come online today. Cameron will say that he wants this too, but this is where the Tory energy policy falls down. Nuclear power – large, immovable and expensive is the antithesis of a free and fair energy market based on renewables.

Building a new generation of nuclear power could dry up all investment in other sources of energy. Nuclear power costs an incredible amount of money. The government estimates that a new reactor will cost £3bn but the latest bids from the companies to build the same reactors in other countries are around twice that. A programme of 10 reactors, as proposed by the government, (and Cameron has said he wants no limit on the number), will cost at least £60bn in upfront costs. This is capital that could be used to invest in renewables.

Of course, the economics of nuclear are only part of the problem. There are also many large and intractable problems associated with nuclear waste. The nuclear waste which will be produced by the new generation of reactors will be hotter and more radioactive than the last generation. It will need to be stored onsite for 160 years and will remain radioactive for the next 30,000. After 40 years of civil nuclear power in this country we still have no safe means of disposing of it. It is an environmental disaster.

The potential for nuclear power to divert resources from renewables, and the dangerous legacy of highly radioactive toxic waste, mean that serious environmentalists cannot support a large programme of nuclear power, and indeed none of the large environmental campaign groups do.

The Conservative party, which just a few years ago became briefly a new champion of environmental causes when they needed to improve their image, now drops serious green causes like a hot stone in the run-up to the election. This is an alarming sign of what Conservative change now means.

For a real change, a change that works for a fair and green Britain, and sound economics, there is the Liberal Democrats.

• Simon Hughes is the Liberal Democrats' shadow secretary of state for energy and climate change.