The Russian-Ukrainian gas row is resolved for now, but it has restarted a debate in Berlin over nuclear energy that may drive a serious wedge into the grand coalition government.

The Christian Democrats (CDU) and its Bavarian sister party, the Christian Social Union (CSU), entered government last November with their traditional rivals, the Social Democrats (SPD), despite not agreeing on everything.

You can't agree to disagree forever, but instead of falling out over healthcare reform or employment legislation, as observers expected, the government's first public row is over energy.

In the coalition agreement, the parties say they disagree on nuclear energy - the CDU/CSU is in favour, the SPD against - but are bound to an agreement signed by the Schröder government and the nuclear plant operators to wind down Germany's 19 plants in the coming years, with the last plant going off the grid by 2020.

However, the stand-off between Moscow and Kiev has left German politicians nervous, as Russian gas covers about 40 per cent of its energy needs. The CDU/CSU has used concerns about reliance on, and reliability of, Russian energy to raise nuclear energy from the dead.

"We are not talking about initiating a renaissance of nuclear energy - that's absurd - but can we afford to switch off 4,000 megawatts of nuclear energy?" asked CSU leader Edmund Stoiber at a party conference on Wednesday.

"Russian gas supplies were reliable in the past. But we need a fundamental rethink how we can exploit energy available in Germany in the longer term," added the economics minister, Michael Glos, also from the Bavarian CSU.

"We have to lay greater emphasis on resources available in Germany . . . we still have a number of nuclear power stations in Germany which, unfortunately, are going to be turned off in a few years time just for political reasons."

Nuclear power is an emotive issue in Germany, particularly since the 1986 accident in Chernobyl. The nuclear plant decommissioning agreement of 2000 was a policy triumph for the Green Party, and something the SPD wants to carry on in the new government.

Written into the new coalition agreement is a plan to generate 12.5 per cent of energy from renewable sources such as wind by the end of the decade, rising to 20 per cent by 2020.

But CDU politicians have hit back, saying these targets are overly optimistic and will not cover the shortfall in German energy production left by the shutdown of nuclear plants, which today generate a third of German electricity.

They are confident that, with their calls to retain nuclear power plants, they have the wind in their sails.

British Prime Minister Tony Blair has spoken out in favour of nuclear power in recent weeks, France recently gave parliamentary backing for a new nuclear plant, while Finland has already begun work on a new plant.

The CDU points out that the coalition agreement refers to the need to "continue and develop further research into the secure running of nuclear power plants". Later on in the agreement, there is talk of a "balanced energy mix" and "an overall plan for energy politics". Watching the developments closely are Germany's giant energy companies, who only half-heartedly signed the decommissioning agreement.

"Without a change in public opinion, one shouldn't think about building new nuclear plants," president of the Vattenfall energy concern Lars G Josefsson told Die Welt newspaper. "But it can always come to that - advanced climate change is a vehicle for that."

The SPD is now coming under pressure to stand up to the CDU or compromise. Environment minister Sigmar Gabriel yesterday suggested that a key component of energy policy in the future would be more awareness of energy usage to save energy.

"If everyone switched off the standby on their machines, we could shut down two nuclear power plants on that alone," he told journalists in Berlin yesterday. He brushed off talk of lifting the death sentence on Germany's nuclear power plants, saying "the idea of independent energy policy in Germany remains an illusion".

SPD colleagues say the CDU is comparing apples and oranges because imported Russian gas is mainly used for heating, while nuclear plants generate electricity.

Germany's energy headache will dominate next week's two-day cabinet meeting and an energy summit Chancellor Angela Merkel has called in early March.

The recent row, and German-Russian energy relations, will also loom large over Dr Merkel's first state visit to Moscow later this month. Close relations between Berlin and Moscow have been a continual cause of concern for the two countries' neighbours.

Poland, Ukraine and the Baltic countries were angry at plans to build a new gas pipeline under the Baltic Sea from Russia to Germany in August.

They feared the new pipeline would allow Moscow, through its Gazprom monopoly, to play politics with gas deliveries to central European countries without affecting western European supply, and this week's Moscow-Kiev spat has heightened those concerns. Dr Merkel has promised to open up the new pipeline to other countries, but observers expect few other concessions to neighbours in the Berlin-Moscow energy alliance, the creation of her mentor, former chancellor Helmut Kohl.

"She might tell them when she goes there about the mess one can make by using energy as a political tool," said Alexander Rahr, specialist in Russian relations at the German Society for Foreign Policy. "But she will do nothing to question the energy alliance between Germany and Russia."

Amidst all their disagreements over energy policy and nuclear power, the CDU/CSU and SPD are determined about one thing: to make sure the energy discussion at home and in Brussels remains an economic one.

German relations with Moscow mean that a wider discussion about common EU energy and foreign policy towards Russia - likely to be raised in March by energy commissioner Andris Piebalgs - might put Berlin in an awkward position.