Euronext, which operates bourses in Paris, Amsterdam, Brussels, Lisbon and Dublin, offered to buy Oslo Bors for 145 Norwegian crowns ($16.58) per share, a 20.8 percent premium to the Norwegian firm's last traded price on Friday.

"We will not need to go to the market, we will use our cash," Stephane Boujnah told Reuters.

Boujnah said his company has long standing contacts and interest in the Oslo exchange, viewing it as a "natural partner" but the opportunity to move forward emerged about six weeks ago when a group of shareholders organised an auction for their stake.

After winning the auction, Euronext secured the support of shareholders representing 49.6 percent of outstanding shares.

Regarding the risk posed to financial markets should Britain leave the European Union without a negotiated framework, Boujnah said Euronext took the risk seriously.

"We hope for the best, prepare for the worst", he said, adding that "considering where we are, less than one hundred days before Brexit, it seems that the possibility of a no-deal Brexit is high".

(Reporting by Julien Ponthus; editing by Sujata Rao)