Borse Dubai, the state-backed exchange operator trying to buy OMX, is today facing a fresh investigation by Sweden's top market regulator, after being found guilty of illegally buying up shares in the Nordic exchange.

The Swedish Financial Supervisory Authority, the FinansInspektionen (FI), spared Borse Dubai a fine despite ruling that a 28.4 per cent holding in OMX it disclosed earlier this month actually constituted a takeover offer.

According to Swedish takeover law, the nature of the holding meant Borse Dubai should have launched a full-blown bid at the time the stake was disclosed, the regulator found.

Borse Dubai, which is vying for OMX with Nasdaq, the American exchange, accumulated a 4.9 per cent stake in OMX. It also struck a series of option deals with hedge funds that gave it the right to own a further 23.5 per cent of the exchange.

Gent Jansson, the FI's acting director-general, said the regulator only decided not to fine Borse Dubai because the exchange subsequently launched its $4 billion bid for OMX, eight days later.

He said the regulator could have imposed a fine of up to Swkr100 million (£7.23 million) but, crucially, could also have blocked the bid.

"We have two alternatives, either we may prohibit and impose a fine or we could abstain from imposing anything. In our decision we have taken into account that they have rectified the breach," he said.

But Mr Jansson added that if Borse Dubai wanted to convert its derivatives holding into a stake in OMX above 10 per cent, it would need to ask the regulator's permission. This would prompt an investigation into whether it would be a "fit and proper" owner of OMX. The regulator's probe would last about 60 days, he said.

"What we have to do according to the [takeover] Act is test whether or not the proposed owner is deemed to be fit and proper and would not jeopardise the stock exchange. All circumstances will be taken into account," Mr Jansson said.

Mr Jansson said Nasdaq had already sought permission to raise its stake above 10 per cent and would find out "in the next couple of weeks" whether the FI would approve or block the move.

The arrival of Borse Dubai on the scene prompted Nasdaq to put its stake in the London Stock Exchange up for sale in order to raise funds, most likely to back an improved counter-bid.

Borse Dubai said: "Borse Dubai acknowledges the conclusions announced by the Swedish Financial Services Authority (SFSA), and we look forward to continuing to work with them as our offer for OMX progresses.

"We continue to believe that our all cash offer of SEK 230 per share for OMX is in the best interests of OMX and its shareholders and represents long-term value for OMX stakeholders by participating in new opportunities in high growth markets as part of the combined OMX-Borse Dubai group."

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