PARIS, May 5 (Reuters) - The chances of a global deal on how multinational companies are taxed have never been higher, although it might take until October to finalise an agreement, the OECD's head of tax said on Wednesday.

The Organisation for Economic Cooperation and Development (OECD) has been coordinating talks among 140 countries for years, and aims to reach a consensus by mid-2021, bolstered by the new U.S. administration's support for a global minimum corporate tax rate.

"The chances of success have in my opinion never been higher because there is a real desire on all sides to wrap up this matter," Pascal Saint-Amans told the French Senate's finance committee.

He added he did not think all issues could be totally resolved by July and that it might take until October to finalise a deal.

The talks aim to make the biggest update to international rules for taxing cross-border commerce for a generation to take account of the rise of internet giants like Google and Facebook.

The discussions are organised around two issues: how to tax multinationals' cross-border digital activities, and a global minimum corporate tax rate.

The Biden administration has suggested a broader scope for the first issue that would go beyond digital business and focus on the profits of the 100 biggest companies that have benefitted most from globalisation.

"I think this American proposal makes sense," Saint-Amans said, adding it would be easier for tax authorities to manage and coordinate.

On the second issue, the Biden administration has proposed a minimum corporate tax rate of 21%, much higher than the 12.5% that discussions had centered around until then.

Saint-Amans said he doubted countries would agree on a rate that high, but added he had high hopes that agreement on a rate close to that could be found. (Reporting by Leigh Thomas. Writing by Benoit Van Overstraeten. Editing by Sudip Kar-Gupta and Mark Potter)