Oil traders have been largely sceptical about the cartel carrying out the three output cuts totalling 4.2m barrels a day it announced since September. But preliminary evidence of lower supplies has now pushed prices higher.

The clearest signal of Opec's success in cutting production came yesterday in a record narrowing of the price spread between lower quality, heavy sour crude – which constitutes the bulk of the cartel's output – such as Dubai oil, and higher quality, light sweet oil, such as the international benchmark Brent oil.

The so-called Brent/Dubai exchange of futures for swaps traded in Singapore at parity for the first time since the instrument was launched 10 years ago, down from a premium of 40 cents the previous day. It traded as high as $5 a barrel before Opec started to cut its production last September.