* MP says ECB bond plan poses "irresponsible" risk to German budget
* Lodges new complaint with Germany's constitutional court
* MPs, legal experts say complaint unlikely to hold up ESM ruling
* Unease with bailouts growing in Germany
By Sarah Marsh and Andreas Rinke
BERLIN, Sept 9 (Reuters) - A lawmaker from Angela Merkel's conservatives has lodged a new complaint with Germany's top court over euro zone bailouts, but legal experts and MPs say this is unlikely to prevent the court from making a key ruling on the bloc's rescue fund this week.
Germany's constitutional court holds the fate of the euro in its hands when it rules on Wednesday whether the European Stability Mechanism (ESM) can go ahead, after already holding it up for several months.
Any postponement of the ruling could unsettle bond markets and exacerbate the three-year-old debt and financial crisis gripping the 17-nation European currency bloc.
Lawmaker Peter Gauweiler said in a statement on Sunday the fund should not be ratified unless the ECB rowed back on its plans to make unlimited purchases of sovereign bonds, since that he said, posed a major risk to Germany's own national budget.
"The ESM - as long as it is constitutional at all - should only be able to enter into power when the ECB has taken back its self authorisation to become a hyper rescue fund," said Gauweiler, one of the critics who filed a complaint with the court against the ESM.
"This possibly calls into question the plans for the announcement of the ruling on the ESM this coming Wednesday."
The ECB has made its plans for potentially unlimited purchases of sovereign bonds contingent on the activation of the ESM, legal experts said. As such, Germany's top court could be considered to be indirectly giving the go-ahead to the central bank's plan if it ruled in favour of the ESM.
Gauweiler argues that the bank's plans raise the risks to Germany's own budget uncontrollably and override its parliament's budgetary sovereignty. The overall risks to its budget from euro rescue measures were therefore becoming "completely incalculable and therefore also irresponsible".
ECB policymaker Joerg Asmussen later on Sunday said the fact all 17 euro zone states must agree on aid packages for countries whose bonds the ECB will buy provides political legitimacy for the central bank's plan.
Moreover, the bank deems the plan necessary to maintain price stability, given disturbances in the markets, and must act independently of politics - a principle Germany has traditionally championed, Asmussen told television channel ZDF.
Markus Kotzur, law professor at Hamburg University, said the German court could not determine what was within the ECB's remit as this was a matter for the European Court of Justice. But it may have to take the ECB's plans into account in its verdict.
"The court would have to think very seriously about delaying its decision on the ESM because delaying it to the start of September had already been seen as critical, people said there was an urgency to act and without the ESM the euro zone might collapse," Kotzur said.
"But the court is probably not completely surprised by this complaint and already has included these considerations in its ruling," he added.
Michael Stuebgen, speaker on European policy for Merkel's conservatives, said the ECB's announcement of its plans last Thursday did not herald a new crisis measure to be contested afresh as, "even to date there had been no real limit on the purchases of sovereign bonds".
Unease with Europe's strategy on dealing with its debt crisis is growing in Germany, Europe's paymaster.
Gauweiler is a member of the Christian Social Union (CSU), Bavarian sister party to Merkel's Christian Democrats (CDU), which in particular has been ratcheting up its tough line on the euro zone crisis ahead of a state election next year.
When the ECB unveiled its plans last Thursday for potentially unlimited purchases of bonds, a small but vocal eurosceptic minority of Merkel's MPs accused it of writing a "blank cheque" to indebted euro zone states, and some threatened legal action to stop the purchases.
The head of the German central bank, one of the country's most respected institutions, was the sole dissenting voice in the ECB's decision. Legal expert Gunnar Beck believes that lent authority to a broader opposition to the plan.
"If the Bundesbank president hadn't spoken out so fiercely I don't think the German public would be as incensed with the ECB as recent polls suggest it is," said Beck, a legal expert at the University of London. "And Gauweiler has further highlighted the issue with his complaint."
Germany's top court cannot simply ignore objections to the ECB's plans, if it wants to be taken seriously, Beck added.
"I do think people are waking up. Now, that doesn't mean the constitutional court will, but it does influence it," he said.
Hundreds of protestors brandished placards with the words "Merkel, who should pay for this?" and "Stop EU inflation and debt union" marched up to the constitutional court in Karlsruhe on Saturday, urging it to reject the ESM.
Legal experts polled by Reuters unanimously expect the court to approve the ESM and budget rules in its ruling on Sept. 12., but they also believe it will impose tough conditions limiting Berlin's flexibility on future rescues.
German Finance Minister Wolfgang Schaeuble was quoted by weekly Bild am Sonntag as saying he was not worried the court might rule against the ESM.
"We were very careful in the creation of the ESM to make sure it did not breach the constitution," he said.
"And one should not forget that the constitutional court has never so far ruled that the course of European integration is against the constitution."