STOCK market regulators are reportedly calling for changes to the deal Diageo has proposed to buy a controlling stake in Vijay Mallya's United Spirits.

The agreement for Diageo to pay around £1.3 billion to buy a 53.4% shareholding under a two-part process was announced last year.

Part of the transaction involves a tender offer for shares, which was due to open on January 7 and close on January 18, but has not yet gone ahead.

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Now there are suggestions the Securities and Exchange Board of India (SEBI) is asking for clauses in the offer to be amended.

The regulator is understood to be unhappy over a clause that allows Mr Mallya's UB Group to sell its remaining stake for the current offer price.

If SEBI believes the option – which can be activated at any point in the seven years once Diageo's stake passes 50% – is a futures contract then that would be seen as a breach of Indian takeover rules.

Last night Diageo said: "We continue to co-operate with SEBI as we address their questions during this review period.

"We have no comment to make on the media speculation around this process."

Diageo has previously stated it hopes to complete the deal by the end of the first quarter of this year.