Pharmaceutical giant Shire stepped up its defence against a £27bn bid from US rival AbbVie yesterday by outlining a long-term plan it believes will convince investors to keep it independent.

The ADHD drugmaker, which shifted its headquarters to Dublin in 2008 to take advantage of Ireland’s low corporate tax rate, has already rejected three bids from Abbvie, the last one valuing it at £46.26 a share. But in a call to investors and City analysts, Shire, chaired by Susan Kilsby, followed AstraZeneca’s lead by giving long-range sales forecasts for its drugs to help prove that AbbVie’s offer undervalues the business.

The company said it plans to more than double sales to $10bn (£5.8bn) by 2020, citing hyperactivity medicine Vyvanse – which is now also being tested as a treatment for binge eating – and dry-eye disease drug Lifitegrast as key areas of growth.

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Chief executive Flemming Ornskov said sales could even exceed this figure because the long-term forecasts do not include revenue from future deals. Shire bought rare disease specialist Viropharma for $4.2bn last year and has hinted that it remains on the lookout for other acquisitions. “The board believes that Shire’s focused growth strategy and efficient cost base will continue to deliver significant shareholder value and patient benefits,” he said. “M&A will clearly add growth to the profile. If I included that in the upside it would be way north of $10bn.”

Mr Ornskov said in the medium term sales should hit $6.5bn in 2016. Shire also highlighted the risks associated with AbbVie’s plan to cut its corporate tax bill by redomiciling in Britain – which was also the intention of Astra’s failed suitor Pfizer. Such ambitions have sparked political concerns on both sides of the Atlantic.

In response, AbbVie raised its own outlook for 2014, citing experimental cancer and multiple sclerosis drugs as its own growth areas. The company said it expects earnings-per-share to be between $3.06 and $3.16 this year.

Pharma analysts at Jefferies predict Shire investors would back a deal priced at about £55 a share. Shares in Shire, which hit an all-time high on Friday, fell 68p to 4,303p.

Mick Cooper, analyst at Edison Investment Research, said: “Shire’s understandably robust defence has credibility given the company’s current strong growth and improvement in operating margins. The targets may appear a stretch, but Shire has dominant positions in very attractive niche markets and these should be achievable.”