On September 13, 2007, Zachary P. Dery appeared for trial on charges of operating while under the influence of alcohol and other traffic violations. The trial judge began the process of empanelling a jury; however, due to the exercise of peremptory challenges and the disqualification of potential jurors for cause, the jury venire was reduced to only five people. The judge informed Dery that the trial could proceed with five jurors *824if he consented. After conferring with his attorney, Dery informed the judge that he was willing to consent to be tried by five jurors.2

The judge then conducted a full colloquy with Dery, after which he found that Dery had freely and willingly consented to trial by five jurors. Dery was not asked to execute a written waiver of his right to a six-person jury, and consequently did not do so. The Commonwealth did not object to proceeding with a five-person jury, or to proceeding without a written waiver. The jury were sworn and the trial proceeded to verdict. Dery was found not guilty.

Several days after the verdict, the Commonwealth moved to return the case to the trial list on the grounds that as a constitutional matter, “a jury of five is not a legitimate fact finder.” Alternatively, the Commonwealth contended that even if such a trial was constitutionally permissible with Dery’s consent, Dery’s failure to execute a written waiver of his right to be tried by a full jury, as required by Mass. R. Crim. R 19 (b), 378 Mass. 888 (1979),3 was fatal to its validity. The judge denied the motion, and the Commonwealth petitioned a single justice of this court for relief pursuant to G. L. c. 211, § 3. The single justice reserved and reported the case without decision to the full court. We deny the Commonwealth’s petition for relief.4

With respect to the Commonwealth’s principal claim, that the trial of the case before five jurors was a nullity as a matter of constitutional law, we refer to our decision in Commonwealth v. Nicoll, ante 816, 820-821 (2008), decided this date, to the contrary.

With respect to the Commonwealth’s posttrial contention that *825Dery’s failure to execute a written waiver of his right to be tried by fewer than six jurors, as required by rule 19 (b), is fatal to the validity of the trial, we conclude that the Commonwealth forfeited any such claim when it failed to object to proceeding on the basis of Dery’s oral waiver, taken after a full colloquy under oath. While a written waiver (of a jury trial) is an important procedural safeguard deemed necessary by the Legislature to preserve trial by jury as a “basic and fundamental right in our judicial system,” Commonwealth v. Osborne, 445 Mass. 776, 780 (2006) (citing G. L. c. 263, § 6),5 it is a safeguard “for the benefit of a criminal defendant” (emphasis supplied). Id. The requirement set forth in rule 19 (b) that the defendant execute a written waiver to be tried by fewer than six jurors serves a similar purpose.6 We fail to see how subjecting Dery to a second trial would safeguard his statutory and constitutional rights to a jury trial, further the purpose of role 19 (b), or further any proper interest of the Commonwealth in this case. Therefore, we decline to exercise our supervisory powers under G. L. c. 211, § 3, and deny the Commonwealth’s motion.

So ordered.