One of the nation's most prominent investors has slammed the competition watchdog's controversial decision to block TPG Telecom and Vodafone Hutchison Australia's $15 billion merger, claiming the government influenced the regulator to protect the national broadband network.

Robert Millner, chairman of Washington H Soul Pattinson and a TPG board member, said the Australian Competition and Consumer Commission's decision had left him "shocked and disappointed". Soul Patts has a 25 per cent shareholding in TPG.

Robert Millner is chairman of Washington H Soul Pattinson. Credit:Peter Braig

"It's very disappointing, I'm amazed the decision was handed down. I don't think there's any basis for it at all other than the government trying to protect the NBN. I don't know what else to think," Mr Millner said.

The regulator's decision was embarrassingly and inadvertently published on Wednesday while the market was still trading. TPG's share price fell by 13.5 per cent to $6.07 after the blunder, wiping about $1 billion from its market value. The share price fell 0.3 per cent on Thursday to $6.05.