Virgin Atlantic is to appeal against the European Commission's decision to allow the owner of British Airways to buy the airline BMI.
IAG, which also owns Spain's Iberia, has been given the go-ahead to buy its smaller rival from Lufthansa for £172.5m.
But Virgin said that the Commission "seemingly ignored all of the strong cases" made against the deal.
The deal is expected to take effect on 20 April, IAG has said.
The sale of BMI to IAG was announced in December and regulatory approval was granted by the Commission on 30 March.
BA will gain 56 more slots at Heathrow airport, although it has been forced to give up 12 slots as part of the deal.
But Sir Richard Branson, president of Virgin Atlantic, criticised the Commission for reaching its conclusion in 35 working days.
"This deal was agreed with lightning speed and we think the number of slots offered is derisory - representing less than a quarter of BMI's former network," he said.
"We will challenge every aspect of this process which if allowed to stand, will undoubtedly damage the British airline industry for years to come.
"The European Commission has seemingly ignored all of the strong cases made by politicians, business groups and airlines, to enable one big company to become even more bloated."
Virgin will bid for all the 12 slots that BA has been forced to give up, the airline added.
IAG has said that the takeover could result in the loss of up to 1,200 jobs.
BMI employs more than 3,600 staff, but reported a £153m loss in the year to 2010.
Based in Castle Donington, in Leicestershire, it operates flights to Europe, the Middle East and Africa.
It has 8.5% of the landing slots at Heathrow, the UK's busiest airport.