Metro Bank shares hit a new low on Monday despite efforts to shut down social media rumours questioning the lender’s financial health.

The bank has been briefing staff on how to reassure customers that their money and safety deposit boxes are safe after a flurry of WhatsApp messages caused queues at some of its London branches over the weekend.

One branch in Harrow in north-west London on Monday contained about 40 customers at midday, with some waiting about 30 minutes to access their safety deposit boxes. Some said they wanted to empty their boxes, where they stored items such as family gold and jewellery.

However, Metro said it was a normal day for the Harrow branch, which “is always busy”. The same branch experienced queues at the weekend as rumours about the bank’s financial health spread. Metro’s share price has sunk nearly 80% since January, when an accounting error was uncovered by the Bank of England’s Prudential Regulation Authority. However, Metro insists that it is on a robust financial footing.

Analysts said the social media and messaging app rumours were only compounding investors’ concerns about the bank’s financial health, with its share price closing down 11% to a new low of 475p per share – compared with a post-flotation high of £40 last year.

A WhatsApp message shared by Metro customers on Twitter at the weekend linked to a BBC story about Metro’s falling share price and urged anyone with a Metro Bank account or safety deposit box to “empty [them] as soon as possible”, claiming the lender was facing financial difficulties and may be “shut down … or going bankrupt.”

Metro said the messages amounted to “false rumours” and that the majority of customers queuing were primarily interested in accessing items stored in safety deposit boxes rather than cash in their accounts. Cash deposits of up £85,000 are covered by law under the Financial Services Compensation Scheme, while Metro stressed that the contents of safety deposit boxes are always the customer’s property.

A Metro Bank spokeswoman said: “We’re aware there were increased queries in some stores over the weekend about safe deposit boxes following false rumours about Metro Bank on social media and messaging apps.

Metro Bank: why are customers worried and are they covered? Read more

“There is no truth to these rumours and we want to reassure our customers that there is no reason to be concerned. We’re a profitable bank, rated No 1 for personal current account service by the [Competition and Markets Authority] and committed to serving our 1.7m customer accounts.”

Russ Mould, investment director at the stockbroker AJ Bell, said the social media “chatter” had “spiralled out of control”, with “memories of the run on Northern Rock in 2007 still in many people’s minds”.

That was the last time a major UK lender had a mass of customers queuing to withdraw deposits as they panicked over the state of the bank’s balance sheet. However, that was before the social media era, with lenders such as Metro Bank now operating in an environment where it is harder to shut down rumours.

Platforms such as WhatsApp allow rumours to be forwarded quickly between close family and community groups. Metro Bank used its Twitter account at the weekend to tweet to customers to assure them that it is stable.

It is not clear whether the source or timing of the WhatsApp messages are being investigated by Metro Bank and it was not able to confirm how much cash may have been withdrawn as a result.

AJ Bell’s Mould said: “Having hundreds of customers queue up to withdraw money certainly hasn’t helped matters, particularly as the business is in the process of trying to raise £350m to alleviate concerns about its balance sheet.

“While Metro Bank has done its best to reassure customers that their money is safe, pictures of one of its branches packed with individuals wanting to cash out is damaging to its reputation and could hurt new customer growth, at least in the short term.”

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Metro hit the headlines in January after it revealed that hundreds of millions of pounds of loans for commercial and buy-to-let property had been wrongly classified as being lower risk than they actually were. Banks are required to put aside more cash to cover their riskier products, to ensure they are protected in the event of a sudden downturn.

Shortly after announcing the accounting mistake, Metro Bank said it would be raising another £350m in funding, only months after raising £300m from investors last July when it ruled out further cash calls.

The bank has denied that the £350m fundraising – which is expected to launch this week – is linked to the accounting errors and has stressed that it is earmarked for future growth.