Adidas, the IAAF's biggest sponsor, has told athletics' world governing body it is to terminate their sponsorship deal four years early, the BBC has learned.

The sportswear giant informed the IAAF of its decision - understood to be a direct result of the doping scandal sweeping the sport - earlier this week.

Sources have told the BBC the move will result in tens of millions of dollars in lost income to the IAAF.

It is sure to come as a major blow for embattled president Lord Coe.

Neither Adidas nor the IAAF - the International Association of Athletics Federations - have made any comment.

The BBC understands that Adidas informed the IAAF in November it was considering ending their relationship early after a report detailed claims of "state sponsored doping" within Russia.

The report was compiled by an independent commission set up by the World Anti-Doping Agency (Wada).

Earlier this month, the commission's chairman, Dick Pound, delivered a second, damning report that revealed "corruption was embedded" within the IAAF under former president Lamine Diack.

Within days, a decision at the highest level in Adidas was taken to terminate the relationship with the IAAF.

It is understood the German multinational believes the doping revelations in Pound's reports constitute a breach of its agreement with the IAAF.

The 11-year deal was signed in 2008 and due to run until 2019. At the time it was signed, it was reported the deal was worth $33m (£23m).

But sources have told the BBC that the figure is much higher, as much, in terms of cash and product, as about $8m (£5.6m) per year.

This means the projected lost revenue for the IAAF over the next four years will be more than $30m (£21m).

It is not clear yet whether the IAAF will attempt to challenge the decision in court, although lawyers at Adidas are understood to be preparing for such a move.

The withdrawal of Adidas will come as a major blow to the sport - and to IAAF president Coe - in a time of unprecedented turmoil.

Coe succeeded Diack in August last year and has come under pressure following the publication of Pound's report.

Not only did it claim that corruption was "embedded" in the IAAF, it also claimed that leading figures within it must have been aware.

Coe, who won Olympic 1500m gold at the 1980 and 1984 Games, served as one of four IAAF vice-presidents under Diack for seven years.

Yet despite the spotlight on Coe, Pound voiced his support for the 59-year-old Englishman, saying he "couldn't think of anyone better" to lead athletics out of its current crisis.

The Wada reports on state sponsored doping have left athletics facing an Olympic year with major reputational damage to repair.

It is also facing a French criminal investigation into corruption, which is looking into the awarding of every World Championships since 2007, including London's successful bid to host the event in 2017.

It now seems Adidas believes there is too much reputational risk to its brand to continue its association with the IAAF.

Adidas has also expressed its displeasure at the corruption scandal that continues to engulf Fifa, although it remains world football's governing body's oldest commercial partner.