Gina Rinehart-backed Sirius Minerals has put the spotlight on her Hancock Prospecting and Rio Tinto in suggesting a white knight could emerge to help finance the burgeoning cost of its plans for a potash mine in Yorkshire.

The London-listed stock has taken a big hit since it was revealed that cost forecasts for the fertiliser project had blown out by $US400 million-$US600 million to as much as $US4.17 billion.