A US firm has confirmed its interest in Hampshire drugs company Shire after details of takeover approaches worth as much as £27 billion were revealed.

Shire, best known for its attention deficit disorder drugs, rejected the merger proposals tabled last month by AbbVie and is no longer in talks with the Illinois company.

Founded in the UK in 1986, Shire is listed on the London Stock Exchange but has its tax base in Dublin and sells most of its products in North America.

AbbVie’s pursuit of Shire, which has its UK headquarters in Basingstoke, came at the same time as UK firm AstraZeneca was fighting off a £69 billion takeover offer from US drugs giant Pfizer.

The US firm first approached Shire at the start of May and said that its third and latest cash and shares proposal at the end of last month valued the company at 4626p a share or £27.2 billion.

Shares in Shire, which is one of the biggest companies in the FTSE 100 Index, opened 13 per cent higher at 4234p after AbbVie confirmed the takeover speculation.

It has until July 18 to confirm whether it intends to make an offer.

The pharmaceuticals sector has seen a raft of deal-making in recent months with US companies looking to Europe as a way to reduce their tax liabilities.

AbbVie was formed in January 2013 after it was spun off from Abbott Laboratories.

It employs about 25,000 people worldwide.

Shire’s products provide treatments in areas such as rare diseases and neuroscience.

Shire’s best-selling product is hyperactivity drug Vyvanse, which achieved revenues of $1.2 billion (£703m) last year.

Shire said its board unanimously rejected the latest proposal from AbbVie on the grounds that it fundamentally undervalued the company. It pointed out that a new management team has achieved a “step-change’’ in Shire’s performance, resulting in increased shareholder returns over the last 12 months.

Shire also raised concerns about AbbVie’s plans to redomicile in the UK for tax purposes.

Chairman Susan Kilsby said: “Shire has a long track record of delivering for shareholders and addressing unmet patient needs. Our high-performing management team and focused strategy are producing even stronger results.’’