Iran announced its full return to the global oil market by ordering an immediate increase in production, prompting warnings from fellow Opec members that it risks prolonging the biggest price crash in a decade. Brent crude, the international oil benchmark, fell below $28 a barrel for the first time since 2003, as Iranian tankers loaded with 50m barrels of crude prepared to set sail following the lifting of US and EU sanctions. Marking the end of years of economic isolation, the head of Iran’s national oil company gave the instruction to increase output by about 500,000 barrels a day, further straining relations with regional rival Saudi Arabia, the world’s largest oil exporter.