Fears are mounting that thousands of ordinary shareholders will be denied any say on the Sirius Minerals takeover because of complicated City rules.

Investors in Yorkshire potash miner Sirius will vote on a £405million offer from FTSE 100-listed Anglo American on March 3.

Some shareholders are angry with Anglo's 5.5p per share bid because they would be hit by heavy losses after buying in at a much higher price.

Takeover deal: Investors in Yorkshire potash miner Sirius will vote on a £405m offer from FTSE 100-listed Anglo American on March 3

But Sirius's bosses, including founder Chris Fraser, insist it will run out of money and go bust within weeks if the rescue deal does not go through, making it crucial that shareholders' voices are heard.

Anglo needs 75 per cent of the votes to be cast in its favour to seal the deal.

And in documents issued to shareholders, Sirius has said it is important that 'as many votes as possible are cast' so that the ballot is 'a fair representation' of investors' opinion. But many backers hold their shares in nominee accounts through brokers, rather than owning them outright.

The only way for a nominee shareholder to be counted as an individual voter is to apply for a paper share certificate, a long process which involves paperwork and can be expensive.

Unless investors go through this process, which can differ from broker to broker, giant shareholder platforms such as Hargreaves Lansdown will only count as a single shareholder when the votes are counted, even though they look after the stakes of many people via nominee accounts.

Cliff Weight, director at investor campaign group Sharesoc, said: 'We're very worried that individuals find the whole process too complicated and not worth the time it takes and as a result are losing out.

'The voting process is an arcane system developed when we just had paper and postage snail mail, and it has not been updated for the times.'

He says up to 90 per cent of private investors could hold their shares through nominee accounts.

And at the last annual meeting votes representing a mere 34 per cent of investors were cast, meaning that few regularly take part and are likely to understand the process for such ballots.

Weight added: 'The law needs to be changed and the culture needs to be changed to make this more accessible. It's about democracy and in this case it's shareholder democracy.'

Sirius's bosses, including founder Chris Fraser (pictured), insist it will run out of money and go bust within weeks if the rescue deal does not go through

Some of Sirius's big institutional backers, including Jupiter Asset Management and more recently the hedge fund Odey Asset Management, have blasted the deal.

Jupiter wants Sirius to keep seeking other deals, while Odey last week used its 1.3 per cent stake to lobby Anglo to pay more.

Sirius's share price has fallen to 5.2p, below the 5.5p offer price, amid mounting opposition.

For some companies, splitting out private investors' votes from those cast by institutions is less significant because individuals will not own a high percentage of shares.

But at the last count Sirius had around 85,000 retail shareholders, many of them local people who feel emotionally invested in the mine. They own as much as 50 per cent of its stock.

The deadline for lodging proxy forms, or those through nominee accounts, is on Friday. But many brokers have their own internal deadlines earlier this week that investors need to meet.

Hargreaves Lansdown, for example, has set its deadline for tomorrow. It represents shareholders who hold a staggering 16.5 per cent of the stock.

On investment forums, some users are saying they are finding the process confusing and are worried they could miss important deadlines.

Sirius shareholders have access to a phone helpline open 9am to 5.30pm from Monday to Friday to assist with any inquiries.

It comes as the takeover yesterday got the green light from competition regulators in Brazil.

Sirius has been struggling since September, when it scrapped a £400million funding round needed to unlock more cash to complete the part-built Woodsmith Mine.

The site, in the North York Moors national park, will extract polyhalite, a fertiliser. The deposit Sirius will tap into was formed around 250m years ago more than 1,000m below the ground.

Sirius estimates there are 2.6bn tonnes of polyhalite, enough for 100 years of mining. Woodsmith, it says, will pump billions of pounds into the economy, bring 1,000 skilled jobs to one of Britain's most economically deprived areas and change the lives of hundreds of landowners, who would pocket millions from land rights deals.