The proposed $15 billion merger of Vodafone Hutchison Australia and TPG Telecom has been blocked by the competition regulator in a dramatic intervention in the telecommunications market that the two companies have vowed to challenge in the Federal Court.

The unexpected decision was accidentally published on the Australian Competition and Consumer Commission's website on Wednesday afternoon a day ahead of schedule, caused TPG's share price to plummet 13.5 per cent. ACCC chairman Rod Sims called the error "extremely unfortunate" and promised to conduct a full investigation.