International crude oil markets slumped on May 31 after US President Trump proposed tariffs on all goods from Mexico "until the illegal immigration problem is remedied". The falls were triggered by the increasing threat of global recession and subsequent effect on oil demand.

Global equity markets were already falling and jittery about the potentially damaging trade war between the US and China.

The decision on Mexico would impose tariffs on US energy imports that last year were worth $13bn.

Benchmark Brent crude futures were down more than 10% in May to stand at $64.49/bl at the end of the month.

The losses on international markets were too late in the month to affect UK retail prices and both diesel and petrol prices were up around 1.5ppl on the month, with petrol at the highest level seen in nine months.

Refiners in central and eastern Europe had their own problems and had to cut production of oil products by some 10% in May because of contaminated supply of Urals crude from Russia.

This helped to tighten European spot diesel and gasoline markets, meaning that losses on crude were not filtering through to the transport fuels sector.

Heavy gasoline exports from Europe to the US east coast, as demand picks up there in the summer, also helped to limit supply in Europe.

There can be no guarantees that the trend of lower oil prices will continue. Tensions continue to simmer between the US and Iran after the Islamic Republic was blamed for attacks on oil tankers in the Gulf of Oman in early May. Iran has, in turn, accused the US of "economic terrorism" for imposing sanctions.

"Trade wars are good, and easy to win," Trump tweeted in March 2018. This questionable assertion is now being put to the test. The IMF expects US-China tariffs to reduce global GDP by 0.3% in the short term. A lasting dispute or expanding the scope of trade wars will offer Opec no incentive to consider increasing output when the producers club next meets on June 25 in Vienna.