Aviva may be forced to increase its offer to sway unconvinced shareholders

Aviva's purchase of RAC was in jeopardy this week as only half of RAC shareholders had accepted the proposed deal by the 13 April deadline, experts warned.

The insurance giant was forced to extend the deadline for RAC shareholders to accept the cash and shares offer by a further two weeks in the hope of persuading shareholders of the remaining 48% of RAC shares to sell up.

The RAC and Aviva boards are now expected to launch a marketing assault on any unconvinced shareholders.

But analysts have suggested Aviva may not go ahead with the acquisition unless at least 75% of the shares change hands.

Experts suggested that Aviva may be forced to raise its offer in order to persuade the remaining RAC shareholders to sell. "Either Aviva will have to revise the offer or the deal is off," said an analyst.

Andrew Hubbard, partner at business advisers and accountants Mazars, said: "The slow take-up of the offer may reflect scepticism on the part of RAC's shareholders about the genuine value of the offer."

Hubbard said Aviva's original offer of 462.5p plus 0.7154 of new Aviva shares represented a premium over the share price at the time of the offer of between 30% and 35%.

"Forty-five percent is normally the benchmark figure shareholders can expect to receive in takeovers of this kind," he said.

Aviva general insurance director Patrick Snowball told Insurance Times: "We are very happy with the total number of acceptances at this stage and are confident the deal will be okay."

RAC's board unanimously accepted the deal, worth an estimated £1.1bn, on 9 April.

But 21 days later valid acceptances had been received for 61,791,991 ordinary shares in RAC - just 52% of the total.