07.34 BST Introduction: Heathrow fire caused by preventable fault, report finds, as Ofgem launches investigation into incident

Good morning, and welcome to our rolling coverage of business, the financial markets and the world economy.

Ofgem has opened an investigation into National Grid after the substation fire on 20 March that led to the closure of Heathrow, as a review found that the root cause of the fire was a “preventable, technical fault”.

This caused a “catastrophic failure” on one of the transformer’s high voltage bushings at the North Hyde electricity substation in Hayes, west London.

The energy regulator said it had launched an official enforcement investigation into National Grid Electricity Transmission (NGET). It will examine the incident, its causes and take further action as necessary.

The final report from the National Energy System Operator said the fault had first been detected seven years ago but had not been fixed.

This review has seen evidence that a catastrophic failure on one of the transformer’s high voltage bushings at National Grid Electricity Transmission’s 275kV substation caused the transformer to catch fire. This was most likely caused by moisture entering the bushing, causing an electrical fault. An elevated moisture reading in the bushing had been detected in oil samples taken in July 2018 but mitigating actions appropriate to its severity were not implemented.

Ofgem will review whether NGET complied with the relevant legislation and licence conditions relating to the development and maintenance of its electricity system at North Hyde.

The regulator will also commission an independent audit into NGET’s critical assets and their status to figure out whether the failings at the North Hyde substation were one-off or more systemic across National Grid.

Japanese stocks fell after Donald Trump threatened a 35% tariffs on Japanese imports, in an attempt to pressure Tokyo into making concessions during negotiations that he described as “very tough”. The Nikkei fell by 0.6% after paring earlier heavier losses.

Yesterday, markets shrugged off the US Senate passing Trump’s “big, beautiful bill,” as investors stayed glued to the outlook for US interest rates and trade deals, ahead of the US reimposing tariffs on 9 July.

Federal Reserve chair Jerome Powell, who has come under heavy fire from the president over the Fed’s failure to cut interest rates this year, yesterday told an audience in Portugal that uncertainty over the impact of Trump’s tariffs prevented the Fed from lowering borrowing costs.

The married couple behind the Prax Lindsey oil refinery awarded themselves at least $15.9m (£11.5m) in pay and dividends in the years leading up to its collapse, it has emerged, as the government urged the company’s boss to “put his hand in his pockets” to help workers.

Winston Soosaipillai, who goes by his middle names Sanjeev Kumar, jointly owned the refinery with his wife, Arani, until it plunged into insolvency on Monday.

The failure of the refinery, which is one of only five left in the UK, has put 625 workers at risk and raised fears about disruption to supplies of customers such as petrol retailers and Heathrow airport.

The Agenda

10am BST: Eurozone unemployment for May

1.15pm BST: US ADP employment change for June

3.15pm BST: ECB president Christine Lagarde speech