Analysts said Rio would have to raise its original $3.80 bid to at least $4.60, with some predicting it would go as high as $5.

Trading in North’s stock was near the top of the volume list on the ASX, with 7.6 million of the shares changing hands for a day’s high of $4.64.

Rio Tinto was not commenting on the market speculation.

Earlier in the day, Rio was given the green light by the Canadian government to go ahead with its hostile takeover attempt of North.

This followed the granting of approvals by the European Commission and Australian Competition and Consumer Commission on Wednesday.

Fighting for control of North is viewed as being among the first couple of shots fired in a global rationalisation spree being driven by the low Australian currency - hence cheap share prices in US dollars - and a lack of growth opportunities elsewhere.