US drugmaker AbbVie has been forced to retract comments by its chief executive about shareholder support for its bid for Shire after being caught out by British takeover rules. 

AbbVie's chief executive Richard Gonzalez told Reuters in an interview yesterday he believed major Shire investors were "generally supportive of this transaction". 

This comment was repeated by the AbbVie CEO in conversations with other media. 

Under UK Takeover Panel rules, a company attempting to acquire a rival is not allowed to claim support for its bid unless it has this in writing from shareholders. 

"AbbVie confirms that it has not received any written commitments of support and accordingly retracts the statements," it said today. 

The error is a further example of the strict nature of British takeover rules, which also caused problems for Pfizer during its unsuccessful bid for AstraZeneca earlier this year. 

AbbVie raised its offer for Shire to £30.1 billion yesterday, hoping to win over its reluctant target after three earlier offers were rejected. 

Shire has yet to respond to the latest cash-and-stock offer,which was worth £51.15 a share at July 7 prices, or 11% more than AbbVie's previous proposal. It said yesteday that its board was meeting to consider it. 

Industry analysts said it was unlikely to be enough to get a deal done but it could bring Shire to the negotiating table. Several analysts have valued Shire in the mid-£50s per share or higher. 

However, with AbbVie's shares losing 3% yesterday, the actual value of the latest offer has fallen to £50.19 a share, which analysts said was inadequate and was likely to be rejected by the Shire board. 

There are worries that the two sides may not manage to reach a deal, as happened with Pfizer-AstraZeneca, prompting volatility in Shire shares, which fell back sharply yesterday after initially rising on news of the raised bid. 

AbbVie may have some room to offer more but it is likely to be constrained by a desire to maintain its investment grade credit rating. 

The US group is eager to buy Shire both to reduce its tax bill by moving its tax base to Britain - a tactic known as inversion - and to diversify its drug portfolio by adding Shire's specialised drugs for hyperactivity and rare diseases.