Pan-European stock market operator Euronext today officially launched its all-cash $729m bid for Oslo Bors.
The move comes just hours after the Norwegian stock market operator's board had said it had found alternative bidders.
Euronext offered to pay 145 Norwegian crowns per share of Oslo Bors, which would value the whole company at 6.24 billion Norwegian crowns ($729m), or about €625m.
The offer is set to expire on February 11.
Euronext had unveiled its plan to launch a tender offer for Oslo Bors in late December and said shareholders representing a little more than 50% of Oslo Bors' shares had already pre-committed to sell at the price offered.
The move to proceed with the offer comes after Oslo Bors's board said it received alternative bids and would issue a recommendation by late February.
In reaction to the initial offer made by Euronext, Oslo Bors said it would search for other potential buyers.
Oslo Bors board told its shareholders to wait for its advice before committing to the Euronext offer.
"The board is working to find the best solution for shareholders and the Norwegian capital market," Oslo Bors spokesman Per Eikrem said today.
"We don't yet know which offer may ultimately be the best. We're trying to find that out now," he added.
It is not clear whether the shareholders who already pre-committed to sell could take another offer that would outbid Euronext.
The Paris-listed stock market operator already owns 5.1% in Oslo Bors.
Euronext, which runs exchanges in Dublin, Paris, Brussels, Amsterdam and Lisbon, is looking to expand its portfolio.
But opportunities are scarce as market operators either already belong to groups such as the London Stock Exchange or Nasdaq, or because their shareholders want to remain independent.
Mega-mergers have also met opposition from competition regulators, who have previously blocked a planned tie-up between Deutsche Boerse and the London Stock Exchange.
Euronext said it made its offer within a "compact auction" process.
But Olso Bors' chief executive Bente Landsnes said last week that potential buyers she did not name had told her they were not invited to bid in the auction.
A deal would match both Euronext's strategy of bolt-on acquisitions and its aim to diversify its revenues from share and derivative trading, given Oslo Bors's leading position in seafood derivatives as well as oil services and shipping.