CIPR immediate past president Lionel Zetter, PRCA public affairs committee chair Rod Cartwright and APPC chair Gill Morris were due to appear before the MPs on Thursday, shortly after PRWeek went to press.
The Commons committee is pressing ahead with its inquiry into the public affairs industry.
Zetter, Cartwright and Morris were expected to say self-regulation is working and a statutory system of regulation is not needed.
But they were also expected to attack agencies such as Bell Pottinger and DLA Piper that refuse to sign the existing voluntary industry-wide ethical codes.
Last week's session with the MPs John Greenway, Nick Harvey and Ian McCartney was cancelled and will now be at a later date.
www.prweek.com/uk for latest news from this week's session.