Gerhard Schroeder has defended his role in a Gazprom-led pipeline scheme after it emerged his government agreed to guarantee finance to the Russian firm.
Mr Schroeder says he knew nothing about the loan guarantee
German opposition politicians have called on the former chancellor to sever links with Gazprom, claiming his involvement is a conflict of interest.
Before Mr Schroeder left office, his government agreed to underwrite a 1bn euros ($1.2bn; £698m) loan to Gazprom.
Mr Schroeder said he was unaware of the decision, or any discussions about it.
In an interview with German newspaper Handelsblatt, Mr Schroeder said he had "no knowledge" of the proposed loan guarantee for a consortium building a huge gas pipeline between Siberia and Germany.
Mr Schroeder joined the supervisory board of the pipeline project - in which Gazprom is the main investor - as chairman late last month.
His decision to join the Gazprom-led scheme so soon after leaving office in November has been heavily criticised by some German politicians.
While Chancellor, Mr Schroeder was a staunch ally of Russian President Vladimir Putin.
On Saturday, it emerged that a ministerial committee approved the loan guarantee to the state-controlled Russian firm on 24 October, weeks before Mr Schroeder stepped down from office.
The guarantee would have required Berlin to repay the loan to a group of German banks if Gazprom had not been able to do so itself.
The Russian firm decided against accepting the loan guarantee, originally intended to help finance a section of the pipeline between the Yuzhnoye Russkoye gas field and St Petersburg.
In a robust defence of his position, Mr Schroeder said he had not been involved in any discussions about the pipeline's financing while in government.
"I did not know about these proceedings," he said.
"But the key point is that Gazprom has already said that it has not and will not accept the financing option put forward by the banks."
The 1,500 mile pipeline will run from the Baltic Sea to Germany
Mr Schroeder added that he had only decided to take a job with the consortium in December, after leaving office.
The German Green Party and the Free Democrats have called on Mr Schroeder to resign his position with the consortium.
Free Democrat leader Guido Westerwelle said Mr Schroeder had to sever his links with Gazprom in order to "keep some credibility".
According to the Sueddeutsche Zeitung newspaper, the German Parliament was only told of the ministerial decision to approve the guarantee at the end of March.
Germany and Russian signed the 4bn euros gas pipeline agreement in September, just before last year's German elections.
Besides Gazprom, German firms E.ON and BASF are also investing in the huge project.