Sirius Minerals, the company building a £4bn mine under the North York Moors, is to be sold for £405m after a rebellion by irate small investors against the cut-price deal failed.

Following a fractious shareholder meeting, at which security guards were drafted in to monitor “credible threats” against directors, investors ultimately voted narrowly in favour of a sale to global mining firm Anglo American.

The UK’s first major deep mine in 40 years now appears likely to go ahead.

Sirius chairman Russell Scrimshaw said: “The positive outcome from today’s meeting secures a return for shareholders, and provides greater certainty in terms of safeguarding the project, protecting the jobs of our employees, and allowing the community, region and the UK to continue to benefit from the project.”

But many small shareholders stand to lose pension money and savings after investing in the company at far higher prices than the 5.5p per share Anglo is paying.

Many individual shareholders had bought shares priced at 20p or more, following bold promises from the chief executive, Chris Fraser, who painted a picture of an asset that would churn out billions of pounds in profit over more than 50 years.

At the meeting immediately preceding the vote, some branded members of the board “stupid” and called the takeover a “bloody disgrace”.

Executives urged them to put their anger at the collapsing share price aside and vote for a sale to Anglo, which swooped in after Sirius failed to raise enough funds to complete the project and warned it would soon run out of money.

Despite a warning that the alternative was an administration process that would leave them with nothing, a cohort of investors had steadfastly refused to back the deal, raising fears that a project expected to create 1,000 jobs would flounder.

The vote needed support from 75% of the shares by value, receiving 80% support. It required a majority of individual voting investors and won 62% support.

The result comes nearly a decade after Sirius Minerals first unveiled plans to build a mine under the North York Moors to exploit a rich seam of polyhalite, a mineral that can be used as a potent fertiliser.

It built popular support for the plan by rallying 85,000 small investors to fund an ambitious undertaking that involves sinking two mine shafts and building a 23-mile underground conveyor belt to take polyhalite to Teesside for export.

Its share price tumbled after a £400m bond issue failed, a crushing setback the firm blamed on Brexit sending a chill through debt markets and a refusal by the government to offer loan guarantees.

Earlier this year, Sirius announced that it had no funding alternatives and had no choice but to accept a takeover offer from Anglo American at 5.5p per share.

At at investor meeting at the Honourable Artillery Company in London, Fraser and the board chairman, Russell Scrimshaw, repeatedly advised shareholders that they should accept Anglo’s takeover or face the company’s almost certain collapse into administration by the end of the month.

“This isn’t a great price but it is the price,” said the chief executive, who acknowledged that his inability to secure financing to complete the mine was the “greatest failure of my career”.

Fraser also criticised the prime minister, Boris Johnson, for refusing to issue a loan guarantee that could have helped Sirius secure $2.5bn (£1.9bn) in funding from the Wall Street bank JP Morgan.

“As much as the prime minister makes these statements about wanting to support the north, this is the poster child for what they should be supporting,” he said. “They chose not.”

Despite directors’ insistence to the contrary, some investors were holding out hope that voting down the takeover would smoke out a white knight buyer ready to submit a higher bid before it collapses into insolvency.

Some sold all but one share, just so that they could have a vote at the meeting.

Paul Thurgood, a gardener, said: “We’ve been cut out of the equation with a low-ball offer and I feel that there are other options out there.

“It’s not that I don’t want the project to go ahead but I’ve got hope that something will turn up in the next few weeks. I’ve already lost a lot anyway.”

Michael Holland, a private investor, said there was “nothing Machiavellian” about the company’s failure to find financing but that Fraser had been presented as a “financial whizz-kid” able to raise funds and had “simply not lived up to the billing”.

During the meeting he said voting against the deal would be “delaying the inevitable”.

Sirius Minerals has already spent $1.5bn on the Woodsmith mine, near Whitby. The company said Anglo would need to invest a further $3.1bn to get the mine up and running within two years.