As we warned a few months ago, Google Merchant Center is going to start requiring unique identifiers for most products uploaded to Google Product Search (Google Shopping).

This requirement started officially at midnight last night, May 3, 2011. Google blogged it yesterday once more to notify Google Merchants of the change.

Although the requirement is now officially being enforced. Google said it is not being full enforced on all merchants. Google will begin rolling out the requirement now and by June 20th or after, Google will begin disapproving data product feeds without this required field.

Google’s Ronald also posted this in the Google Merchant Center Help forum, where he said:

Again, I’d add the unique identifier field asap, if you have not. But it seems you still have a month or so to add it until your feeds are disapproved.

Forum discussion at Google Merchant Center Help.