Pressure is growing on Twitter’s chief executive Jack Dorsey to step down after it was alleged that up to 48m of its accounts — equal to 15% of its users — were robots not people.

Twitter was last week hit by a report from professors at the University of Southern California and the University of Indiana that estimated that as many as 15% of the social media site’s 319m users were not human. That is nearly twice the company’s own estimate that up to 8.5% of its accounts are managed by “bots”.

The charge is particularly damaging for Twitter because it is already struggling to convince advertisers to return to its platform. Dorsey, 40, is one of the site’s co-founders and returned as chief executive