UPDATE 26 March: Retail consultant Mary Portas has warned that the department store chain John Lewis is in danger of losing its “soul” following a proposal to dilute its partnership model.

Her comments were echoed by one of its former bosses Andy Street who said such a move would be a “tragedy”.

The partnership’s chairwoman Dame Sharon White has proposed selling a minority stake after it reported a £234m loss. The staff bonus has been cancelled and job cuts appear likely.

Ms Portas said John Lewis was part of “our collective cultural identity” as a nation and she has written an open letter to Dame Sharon and new chief executive Nish Kankiwala expressing her concerns.

“You are custodians of one of the most valued, loved, and trusted retail brands this country has,” Ms Portas writes.

She said John Lewis and its supermarket chain, Waitrose, were part of the “fabric of everyday British life”.

On Sunday, Mr Street, who was managing director from 2007 to 2016, added: “It would be a tragedy if that occurred because I think John Lewis goes a bit beyond a shop.

“You can buy the same television in other places in truth, but John Lewis was about a way of doing business, showing the market there was a better way almost and that’s potentially under threat.”

Mr Street urged the retailer’s leadership to think about what was at the heart of the business, “what makes it special and hold onto that”.

In her letter Ms Portas says: “Your task isn’t to turn around just another mediocre retailer under threat of going under. You’re fighting to save part of our collective cultural identity.

“But what’s worrying me is that you might think your fight is purely financial. It’s not.”

Describing the battle as more nuanced, Ms Portas said what laid ahead was about “the soul of your brand.”

“Somehow, in recent years, you’ve let go of the soul.”

Her letter says: “What we want is to come to you when we’re expecting our first baby and panicking about buying a cot… when we’ve finally made it onto the property ladder and want curtains and a sofa bed that’ll last”.

But she warned that was “being slowly chipped away. From loud headlines to daily whispers”.

Ms Portas said she appreciated the partnership needed “money men” but added: “Never forget to balance all that with the commercially instinctive, reactive, creative, retail-born folk in the business. It’s an art. Not a science.”

She urged the group to recommit to the principles it was founded on – common ownership.

“We know British brand history is littered with those who tampered with the crown jewels of their identity… British Airways clings on. Rest in peace Laura Ashley. And dear Woolworths.

“But failure for John Lewis and Waitrose is not an option. We can’t lose you too.”

In response, Ms White said it is the “biggest privilege of my life to be custodian of the Partnership”.

“I am here to ensure that it not only survives, but thrives for generations.”

She said 100 million customers visited its department stores last year, a third up on the previous year and half a billion customers visited the App and website.

“We want our brands to continue to grow,” she said. “We’ve always been open to new partnerships with investors or likeminded companies to share in our growth.

“I will not rest until the partnership is restored to full health,” she added.