German Chancellor Angela Merkel has rejected an offer for a second TV debate with her Social Democrat (SPD) rival Martin Schulz.

"Angela Merkel enjoyed taking part in the TV duel. The format worked out well. And she is leaving it at that," an official from Merkel's Christian Democratic Union (CDU) party said.

Opinion polls show that Merkel is firmly on course for re-election, with Schulz trailing her by around 14 percentage points.

Read more: Angela Merkel has a huge lead in polls ahead of German election - but are they accurate?

Schulz challenged her to a second debate in a letter. He argued that in the first debate the two had failed to address voters' key concerns related to the digital economy, the future of the labor market, pensions and education.

Schulz criticizes rejection

Schulz, on Wednesday, reacted sharply to the manner in which his offer was turned down.

"I am amazed that I write a letter to the Chancellor's office addressed personally to the chancellor - and it's the [CDU headquarters] that comments on it and rejects it," Schulz said on the campaign trail.

"I have always maintained that the chancellor's office is not the CDU headquarters, but an institution that serves the country," he added. "Obviously, that's not the case."

Schulz said his offer was still on the table and that he was waiting for a response from Merkel herself.

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SPD's Andrea Nahles, who is Germany's labor minister, accused the conservatives of cowardice for not allowing a second debate to discuss issues like health, education and pensions.

"In my view, this is weak of Mrs. Merkel," she told Reuters news agency.

Schulz had failed to dent Merkel's popularity after the first TV duel on September 3, when the two leaders debated migration, Turkey and other foreign policy issues. Both of them agreed on many policy areas.

According to viewers, Merkel made the more convincing case to lead Germany for the next four years in the debate. Some 55 percent of the viewers surveyed by German broadcaster ARD said the chancellor outperformed her rival.

Forty-nine percent of viewers viewed Merkel as the more credible over the course of the debate, compared to just 29 percent who favored Schulz.

ap/sms (Reuters, dpa)