March 28 (Reuters) - Malteries Soufflet, a branch of French agribusiness InVivo, made an offer worth $1.5 billion ($998 million) for United Malt Group, the companies said on Tuesday, sending the Australian company's shares up 35%.

The 100% takeover would make InVivo the world's top malt producer and double the size of its malt business three years earlier than planned, InVivo Chief Executive Thierry Blandinieres told Reuters.

It signed an agreement in January to take over Belgian malthouse Castle Malting, one of the world's oldest producers of the beer ingredient and which is also a big player in the craft beer market.

"We would already have a share of 20% of the malt market with great growth potential ahead of us," Blandinieres said.

A deal would lead to potential combined annual production capacity of 3.7 million tonnes of malt, InVivo said.

InVivo is also looking at external growth in malt in eastern Europe, India and Asia, Blandinieres said.

The A$5 per share non-binding offer represents a 45.3% premium to United Malt stock's last close at A$3.44.

Shares in United Malt soared to A$4.64 in their biggest intraday percentage jump when they returned to trading after being halted on Monday. They finished 30.8% up at $4.50.

United Malt disclosed it first received an offer of A$4.15 per share in December followed by several others from the French suitor. The latest indicative proposal was made on March 14.

United Malt is the world's fourth-largest commercial maltster, producing bulk malt for brewers, craft brewers, distillers and food companies. The company has processing plants in Canada, the United States, Australia and the United Kingdom.

United Malt's main shareholders are Australia's GrainCorp and investment firm Tanarra Capital.

InVivo acquired Malteries Soufflet as part of its takeover of agribusiness peer Soufflet in 2021, with the help of investors including KKR and Credit Agricole. These would keep a similar share after InVivo's planned capital increase, Blandinieres said.

Blandinieres said he was confident that Malteries Soufflet's offer would be approved by the board. The deal would also lead to a delisting of United Malt.

InVivo is a combination of nearly 200 farmer cooperatives that Blandinieres would like in future to rival global crop merchants such as Archer Daniels Midland Co and Cargill. ($1 = 1.5038 Australian dollars) (Reporting by Harish Sridharan in Bengaluru Editing by Subhranshu Sahu, Louise Heavens and David Goodman)