The Environmental Protection Agency said on Wednesday it was again delaying its final rule on smog limits, with the rule now expected by the end of July 2011.

This is the third time the agency has delayed the smog standards, originally slated to be finalized in August.

The initial standards proposed near the start of this year would limit ground-level ozone, or smog, to between 60 and 70 parts per billion measured over eight hours.

The proposal was stronger than 2008 standards the Bush administration set. Environmental groups criticized those for being less stringent than government scientists recommended.