The activist investor campaigning against the £1.1bn US takeover of the British software specialist Blue Prism has claimed a “critical mass” of shareholders are lining up to block the deal.

Coast Capital said it was in talks with around 30pc of shareholders who had raised doubts about Blue Prism’s sale to the buyout firm Vista.

In September, Vista agreed the £11.25-a-share deal with Blue Prism, which specialises in automating repetitive tasks. Shareholders are due to vote next week, with Vista needing three quarters to back the buyout.

Coast’s founder James Rasteh said: “Coast Capital is in discussion with shareholders cumulatively holding over a 30pc stake in the company who are unhappy with Vista’s highly inadequate bid and are assessing their final vote accordingly.”

In a recent letter to the company, he said the deal was facing “increasing resistance by a critical mass of investors”. Coast claims that the £1.1bn takeover undervalues Blue Prism, that it has been subjected to a rushed sale, and that the company has been mismanaged, pushing down its value.

David Brown of Hawk Ridge Capital, which owns a 3.5pc stake in Blue Prism, said the deal was likely to fail. “I think shareholders are likely to reject the offer. “Many investors have reached out to us and expressed their disappointment.”

Vista plans to merge the firm with US rival Tibco and has said up to 475 of the combined workforce – around 10pc of staff – are likely to lose their jobs.

Aim-listed Blue Prism floated in 2016 and shares soared to more than £25 in 2018, but have fallen heavily since. Shares have fluctuated above the offer price in recent days. Vista did not comment.

A Blue Prism spokesman said: "At announcement, the Vista offer received tangible support through irrevocable undertakings and letters of intent from shareholders including four of Blue Prism’s top-ten investors.  

"Since the announcement, the company has engaged with a significant majority of its shareholders and stakeholders, including ISS, who has recommended in favour of the transaction.

"The board believes the Vista offer delivers immediate and certain value for Blue Prism shareholders without the execution risks associated with the necessary strategic investments envisaged, is superior to the company’s standalone alternatives and therefore unanimously recommends that shareholders vote in favour of the scheme."