* German govt spokesman rejects charges of secret pact
* Merkel's party loses support after nuclear extension
By Erik Kirschbaum
BERLIN, Sept 10 (Reuters) - The German government denied on Friday charges that it had made a secretive deal to cap safety costs for utility firms as part of an agreement to extend the lifespans of nuclear power plants.
Chancellor Angela Merkel's spokesman Steffen Seibert dismissed accusations by the opposition Social Democrat and Greens parties that the government made a secret pact limiting the costs of safety upgrades to 500 million euros per reactor.
"There's nothing to hide," Seibert told a news conference after German media reported the existence of a document outlining aspects of a deal in which utilities may cut renewable energy spending if reactor upgrades exceeded 500 million euros.
"There's no secret deal -- security is non-negotiable," he added. "It's more important than all financial considerations." Seibert said details of the broader agreement were released days later because both sides closely examined the framework deal.
Merkel's centre-right government agreed on Sunday to extend the lifespans of 17 nuclear power plants by about 12 years from the current shut down date of 2021.
It is the most controversial decision Merkel has made because most Germans oppose extending nuclear power. An opinion poll on Friday also showed 61 percent are opposed to extending the use of nuclear power and only 33 percent support it.
Merkel's conservatives slumped one point in the poll to 32 percent -- one ahead of the SPD. The Greens, who are leading the opposition to nuclear power, rose a point to 17 percent.
"This is a dirty deal," said Greens parliamentary leader Renate Kuenast.
SPD chairman Sigmar Gabriel added: "Never before has a German government so audaciously sold off public safety."
The nuclear issue contributed to Merkel's Christian Democrats (CDU) losing control of North Rhine-Westphalia in May and could cause the CDU to lose Baden-Wuerttemberg in March. The last SPD-Greens government passed a shutdown law in 2002.
The SPD and Greens plan to challenge the nuclear extension in court and vow to rescind it if they return to power in 2013.
Tens of thousands are expected to take part in a mass anti-nuclear rally in Berlin on Sept. 18.
The agreement to extend nuclear power -- a cornerstone of Merkel's broader energy strategy -- was reached on Sunday after months of wrangling within her coalition and public opposition.
The new details released on Thursday showed that if security upgrade costs exceed 500 million euros for each reactor the utilities would be able to reduce the corresponding amount of funds they promised to pay to boost renewable energy.
The government has previously said security upgrades could cost 1 billion euros per reactor.
The utilities -- E.ON, RWE, EnBW and Vattenfall -- would also be able to reduce renewable energy investments if a nuclear fuel element tax worth 2.3 billion euros per year is extended by 2016. (Editing by Charles Dick)