BUCHAREST, Jan 15 (Reuters) - Romania's Constitutional Court on Wednesday struck down a draft law that would have boosted deputies' immunity from corruption charges, saying its provisions went against the rule of law.

The ruling overturns a decision in December by parliament's lower house to approve the bill, a move that was sharply criticised by the country's president, opposition politicians, pro-democracy groups and Western diplomats.

"The provisions ... break articles of the Constitution that refer to the rule of law," the court said in a statement.

The bill had stipulated that members of parliament could no longer be subject to investigation under corruption allegations linked to public office - such as abuse of power or conflict of interest - due to a change in their status.

The supreme court and opposition MPs challenged the law at the Constitutional Court. The bill now returns to parliament, where it will be debated further.

The European Union has repeatedly raised concerns about a failure to tackle high level graft in Romania and Bulgaria, its two poorest members which have been prevented from joining the passport-free Schengen zone over the issue since their entry.

Romania has also had its justice system under special EU monitoring since its entry in 2007.

Judicial data shows 28 Romanian lawmakers are on trial on corruption charges or serving jail terms, while hundreds of mayors, deputy mayors and councillors have falsified statements, faced conflicts of interest or had wealth they could not explain.

Former Prime Minister Adrian Nastase was sentenced last week to four years in prison for taking bribes, putting him back behind bars just months after he finished serving a previous term.

The leftist politician has repeatedly denied any wrongdoing, and said the cases against him were politically motivated.

(Reporting by Luiza Ilie; Editing by Mike Collett-White)

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