The gap between the price of the main US and European oil benchmarks reached its widest level since August on Tuesday.

Nymex June West Texas Intermediate fell $2 to $82.20 a barrel while ICE June Brent, the North Sea benchmark, fell $1.23 to $85.60. This left the spread between the two above $3 a barrel.

The divergence reflects rising levels of oil storage at the Nymex crude delivery point of Cushing, Oklahoma and the US Midwest. Cushing inventories have risen five straight weeks to 34.1m barrels as of April 16, according to the US Department of Energy. Last week they rose a further 1.1m barrels, according to data provider Genscape. The US government updates its Cushing reading on Wednesday.

The growing stocks have depressed the price of US oil for sale soon relative to oil for later delivery – known as “contango” in industry parlance.

“The higher the level of inventories, the higher the associated contango,” said Harry Tchilinguirian at BNP Paribas in London.

It also heightens costs for passive investors in commodities, who have to pay a premium each time they buy later-dated futures and sell expiring ones.

The softening of spot WTI prices and its weakness compared with Brent has caught many traders by surprise. Analysts who early this year called for oil inventories to decline say the rising tide of Cushing crude will not last. Barclays Capital said the WTI spread was likely to remain under pressure as long as fresh flows of Canadian oil pour into the US. But the bank sees it “more as a short-term phenomenon distorting WTI front spreads specifically, as global oil market balances continue to be in tightening mode”.

JPMorgan, which early this year said contango was at risk of disappearing, anticipates “a much more modest Cushing crude build in the reporting week, turning to draws in the coming weeks, which will reverse the current downward trend in spreads”.

Contango exists in Brent, but not as dramatically. The European oil blend has historically traded at a discount to WTI, reflecting the US’s status as a net oil importer. Goldman Sachs last week recommended traders buy June WTI and sell June Brent futures in a bet that the gap would close.

When Brent traded at a strong premium to WTI last August, some analysts attributed its gains to the threat of a US regulatory crackdown on oil speculation.

But there does not appear to be a recent migration to Brent, traded on London’s ICE Futures Europe exchange. The number of open ICE Brent contracts has fallen 13 per cent in April.

It remains uncertain whether the US Commodity Futures Trading Commission will enact proposed limits on energy trading, as a majority of its commissioners have raised doubts about it.

Elsewhere in commodities markets, cocoa hit a fresh 33-year high. Improving demand from the chocolate industry and a disappointing crop in Ivory Coast, the world’s top grower of the bean used to make chocolate, have seen prices hit their highest levels since 1977. Liffe July cocoa hit an intraday peak of £2,396 a tonne, up 1.6 per cent on the day.