Exterior view of the German Federal Constitutional Court in Karlsruhe. Reuters

Germany's Federal Constitutional Court may delay its long-awaited decision on the legality of Germany's participation in the European Union's permanent bailout fund as a new legal move to challenge the European Central Bank's bond buying programme emerges.

The AFP newswire is reporting that the court in Karlsruhe will hold an emergency session to examine a last-minute filing from Peter Gauweiler, a German lawmaker and member of the Christian Social Union (CSU) in Bavaria.

Gauweiler says the ECB's plan to link its bond buying plan to participation in the fund, known as the European Stability Mechanism, calls into question the constitutional legality of Germany's financial support for the fund.

"The ESM -- insofar as it is constitutionally viable at all -- should only come into force when the ECB has taken back its self-awarded power as a hyper rescue-shield," Gauweiler said in a statement to the German press Sunday.

The AFP source says the Bundesverfassungsgericht will issue a ruling on the last-minute challenge on 11 September. Its decision on the legality of the ESM is slated for the following day.

A German Finance Ministry spokesperson told Reuters that the government is convinced the both ESM and the broader European Fiscal Pact are constitutionally legal.

Gauweiler, a noted Eurosceptic, has consistently challenged the reach of the ESM and Germany's 27 percent funding role, which many investors say is crucial to the survival of the Eurozone.

This is not the first 11th hour challenge to the Court's timetable. In early August, Europolis, a group of German academics led by Markus Kerber, petitioned the court to delay its ruling under a similar challenge was heard by the European Court of Justice, based on a complaint brought by Ireland's Thomas Pringle, an independent lawmaker in the nation's Dáil Éireann.

"As long as the European Court of Justice hasn't taken a final decision on the incompatibility of these treaties, neither the Federal Constitutional Court nor the Federal President must take a decision," Europolis said on its website.

Collectively the ESM and the temporary European Financial Stability Fund form the centrepiece of the EU's "firewall" around the sovereign debt crisis. With a total lending capacity of around €800bn (including prior commitments to Greece from earlier bailouts), the funds have a financial strength equal to the size of Canadian GDP.

Around €80bn in hard capital from the 17 Eurozone members will provide the ESM's base, and it will have the ability to lend up to around €500bn when it's fully operational. The 15 percent capitalisation will be paid by the member states over a three year period (€32bn this year and next and €16bn in 2014). European leaders hope the fund will be strong enough to earn a triple-A credit grade from the three major ratings firms.

Prior commitments from the EFSF are around €192bn based on the EU contributions to bailouts for Ireland, Portugal and Greece. A further €100bn will be earmarked for Spanish banks, but the terms and conditions for the loans, as well as the ultimate size, haven't yet been finalised.

Contributions to the ESM, the permanent form of the fund which will run in tandem with the EFSF for the first few years, will be based on a percentage of the Eurozone member's economic size. For example, Italy is responsible for around 17.9 percent of the €80bn in paid-in capital.

The fund will lend money to ailing economies - and perhaps take direct stakes in banks - in times of distress. Loans from the fund will be senior, in collateral terms, to all existing debts of the borrower expect those owed to the International Monetary Fund (or the pre-committed loans to Greece, Ireland and Portugal).

Short-term loans will be priced at 2 percent premium over the ESM's funding costs (a back-of-the-envelope calculation puts this at around 4 percent). Longer loans will carry a further 1 percent surcharge if they're not repaid after three years.