BERLIN (Reuters) - Germany's centre-left ruling coalition and the main conservative opposition have agreed a draft law to protect its highest court, the Rheinische Post newspaper reported on Thursday, at a time of growing concern about the strength of the far-right AfD party.

       The anti-establishment, nationalist party is already under state surveillance on suspicion of being extremist and anti-constitution. The AfD rejects allegations it is undemocratic and has said the Constitutional Court, which is appointed by parliament, is biased and closely linked with the government.

       The AfD has filed more than 20 cases with the top court alleging its rights are being infringed, according to a Reuters analysis of data revealed last week, a tactic the party's critics say is meant in part to gum up the rule of law.

       The 12-page draft law will incorporate the mandate of the Federal Constitutional Court into the constitution, the Rheinische Post reported, with the aim of enshrining its independence with the following passage:

       "The decisions of the Federal Constitutional Court bind the constitutional bodies of the federal and state governments as well as all courts and authorities."

       The newspaper said the law would also spell out the election process for the court's judges through the lower and upper houses of parliament, their term of office and the age limit.

       "All of these regulations are therefore exempt from change with a simple majority in the future," the Rheinische Post added, quoting the draft legislation.

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       (Reporting by Riham Alkousaa; editing by Mark Heinrich)