The takeover tussle for Nordic exchange group OMX is set to become a full-blown bidding war after Swedish regulators today took no action against Borse Dubai despite ruling that the group broke the law with its stakebuilding in OMX this month.

Borse Dubai, whose $4bn (€3bn) takeover bid for OMX is competing with a lower, agreed bid by US exchange Nasdaq, launched its offer a week after snapping up a 27.5% interest in OMX in a move that drew scrutiny from the Swedish Financial Services Authority.