MILAN, Dec 10 (Reuters) - An Italian court dropped a key corruption charge against Prime Minister Silvio Berlusconi on Friday, invoking a statute of limitations that meant time had run out to convict him of allegations he bribed a judge.

The ruling, read to a packed Milan court, implied that the 68-year-old Berlusconi was guilty of authorising aides to corrupt a Rome judge in 1991 but could not be sentenced because of the time limit.