Shares in Iamgold Corp. were down more than 25 per cent in afternoon trading after the company warned its Côté Gold project will cost more than expected.

The company says the remaining project costs for the mine being built in northern Ontario between Timmins and Sudbury could be between US$1.2 billion and US$1.3 billion.

It also withdrew its 2022 and 2023 Côté Gold project costs guidance and said it will provide a detailed update before the end of the second quarter.

RBC Capital Markets estimated the price tag to complete the mine is up about US$600 million from the company's earlier guidance and that it would be delayed by four to five months.

Iamgold holds a 64.75 per cent stake in the project, Sumitomo Metal Mining owns 27.75 per cent and 7.5 per cent is held by other investors.

Shares in the company were down 99 cents or about 27 per cent at C$2.63 in afternoon trading on the Toronto Stock Exchange.