The asset under a national park in North Yorkshire, northern England, is Britain's biggest mining project and has political resonance as the government works to create jobs in an economically disadvantaged part of the country.

Its future was in jeopardy until Anglo made its 5.5 pence per share bid for a project whose viability some analysts have questioned. But Odey said the offer was too low and seems "to make a mockery of both internal and external audits at Sirius."

In an open letter, Odey, which held a 1.29% economic interest in Sirius as of Feb. 18, said it would only vote in favour of a bid at 7 pence per share or more.

"The lack of 'final' offer, in Odey's opinion, suggests that Anglo American would be willing to bid substantially more for Sirius, with the investment case remaining highly attractive for Anglo American, even at a materially higher bid level," the hedge fund said in the letter.

A spokesman for Anglo American declined comment. Sirius said in an emailed statement that its board was unanimous in recommending shareholders vote for Anglo's bid as it stands.

"We recognise that the current offer does not represent the value that the board and shareholders had previously hoped for. However, given the situation the company finds itself in, we face a stark choice," the statement said.

"In the event of this transaction being unsuccessful there is a high probability that the business will need to be placed into administration or liquidation."

Anglo American agreed last month to buy Sirius Minerals for 405 million pounds in cash, marking the global miner's return to fertilizers.

Sirius had struggled to find financing in private markets and last year scrapped a plan to raise $500 million in a bond sale.

Its shares have fallen by around 75% since the start of 2019 despite a bounce when news of the Anglo American bid broke. By 1200 GMT on Wednesday they were up 3% at 5.19 pence, roughly in line with the UK mining sector <.FTNMX1770>

(GRAPHIC: Sirius shares battered by financing trouble - https://fingfx.thomsonreuters.com/gfx/mkt/13/2269/2237/sirius%20odey.png)

The share price fall has hit many local people who invested in the plan to develop what Sirius has said is the world's largest deposit of polyhalite, a multi-nutrient fertiliser.

By Abhinav Ramnarayan and Barbara Lewis