BrewDog has apologised after former workers alleged there is a “culture of fear” within the business, with “toxic attitudes” towards junior staff.

A “significant number” of ex-employees are said to have suffered “mental illness” as a result of working at the Scottish craft brewing giant, signatories to an open letter said.

The letter alleged the business was built upon a “cult of personality” around its founders, James Watt and Martin Dickie, with “growth at all costs” the overarching focus of the company.

Sent by a group called Punks With Purpose, it said: “Being treated like a human being was sadly not always a given for those working at BrewDog.

“Put bluntly, the single biggest shared experience of former staff is a residual feeling of fear.

“Fear to speak out about the atmosphere we were immersed in, and fear of repercussions even after we have left.”

It alleged that managers treated staff “however they liked, without repercussions – making them feel belittled and/or pressured into working beyond their capacity, and often eventually feeling forced out of the business”.

Co-founder Mr Watt said on Twitter that the letter is “upsetting” but that he will not “contradict or contest” its contents, and instead “listen, learn and act”.

He said in a statement: “We are committed to doing better, not just as a reaction to this, but always, and we are going to reach out to our entire team past and present to learn more.

“But most of all, right now, we are sorry.”

Founded in Fraserburgh, Aberdeenshire, in 2007, BrewDog is recognised as having helped bring craft beer to the mass market.

Its canned and bottled beers are stocked in supermarkets and it operates numerous bars across the UK.

BrewDog is valued at around 2 billion US dollars (£1.4 billion) according to Forbes’ most recent estimates.

In a passage addressed directly to Mr Watt, the letter said: “It is with you that the responsibility for this rotten culture lies.

“Your attitude and actions are at the heart of the way BrewDog is perceived, from both inside and out.

“By valuing growth, speed and action above all else, your company has achieved incredible things, but at the expense of those who delivered your dreams.”