(a) A judge shall not sentence an offender to a term of imprisonment in excess of the maximum sentence authorized by Article 4.5 of Chapter V for an offense or offenses within the class of the most serious offense of which the offender was convicted unless the factors in aggravation set forth in Section 5-5-3.2 or clause (a)(1)(b) of Section 5-8-1 were found to be present. If the pre-trial and trial proceedings were conducted in compliance with subsection (c-5) of Section 111-3 of the Code of Criminal Procedure of 1963, the judge may sentence an offender to an extended term as provided in Article 4.5 of Chapter V (730 ILCS 5/Ch. V, Art. 4.5).
     (b) If the conviction was by plea, it shall appear on the record that the plea was entered with the defendant‘s knowledge that a sentence under this Section was a possibility. If it does not so appear on the record, the defendant shall not be subject to such a sentence unless he is first given an opportunity to withdraw his plea without prejudice.