The Korean shipbuilder, which is looking to diversify away from bulk carriers, acquired 39 per cent of the Norwegian group for $800m last October.
STX Group announced yesterday that it had bought 1.3m shares at Nkr63 a share, bringing its shareholding up to 40.4 per cent, triggering the mandatory buy-out offer.
STX Group said that it would launch a buy-out of Aker Yards as soon as -possible.
A bid price of Nkr63 would value the whole group at Nkr7.2bn ($1.4bn). In the first quarter Aker Yards had a net profit of Nkr41m on revenues from continuing operations of Nkr7.47bn.
STX Group's acquisition of a controlling stake in Aker Yards has raised fears that it could transfer the expertise in cruise ships to its lower-cost Asian yards, leading to the loss of more European shipbuilding jobs.
Cruise ships are one of Europe's last areas of competitive advantage in the industry.
The Finnish government has indicated that it could take a stake in Aker's Finnish yards if it would help preserve shipbuilding in the country.
The French government was so concerned that STX Group could close its two French yards with the loss of 3,000 jobs that it recently reached an agreement with the Korean company to buy back a 9 per cent stake in Aker Yards France.
The French government also has first refusal on the remaining 25 per cent stake in Aker Yards France owned by Alstom, the French engineering group, giving it a potential blocking shareholding in Aker's French subsidiary.
The French yards built the Queen Mary II and are one of the few remaining shipyards in Europe that are large enough to build an aircraft carrier.