The foreign secretary, Boris Johnson, was urged to avoid passing politically sensitive judgments on world events until he was in full possession of the facts after he prematurely blamed Islamist terrorists for the killings in Munich on Friday.

Johnson made his remarks before the identity of the killer – an 18-year-old German citizen of Iranian descent who was obsessed with mass slaughter – had been known.

Although early reports of the attack, in which Ali Sonboly shot nine people dead before killing himself, suggested a gang of three people might be on the loose in Munich in a terror attack reminiscent of the killings in Paris, no definitive information was available and the authorities had not identified a motive for the killings.

Speaking about the attack on Friday while in New York, Johnson told the press that that the “global sickness” of terrorism needed to be tackled at its source in the Middle East.



“If, as seems very likely, this is another terrorist incident, then I think it proves once again that we have a global phenomenon and a global sickness that we have to tackle both at the source – in the areas where the cancer is being incubated in the Middle East – and also of course around the world.”

He added: “We have to ask ourselves, what is going on? How is the switch being thrown in the minds of these people?”

Tom Brake, the Liberal Democrat foreign affairs spokesman, told the Guardian: “Just days into his new role, Boris demonstrates again why it was a huge gamble appointing him. The off-the-cuff remark may suit Have I Got News for You, but it doesn’t the tragedy in Munich. In future, Boris needs to hold his tongue until he is in full possession of the facts. There is too much as stake.”

It follow a press conference alongside the US secretary of state, John Kerry, on Tuesday in which Johnson was questioned by journalists who repeatedly pressed him to explain his past “outright lies” and insults about world leaders, including describing the US president as part-Kenyan and hypocritical.

The Munich tragedy overshadowed Johnson’s other claim in New York that there was such a close relationship of mutual economic dependency between the UK and the eurozone that it was “manifestly in the interests of our [EU] friends to continue with a good relationship”.

London would remain a major financial centre, he predicted, due to its deep capital markets. He argued that there was no sign yet that June’s referendum outcome had prompted global financial companies to leave the City; on the contrary, he predicted that it could become even stronger after Brexit.

“The New York commercial scene regards this as an opportunity for the UK,” he said after meetings with a series of senior American business, financial and political leaders. “If we get this right, this can be a fantastic thing.”

Johnson insisted that there was no chance of the Brexit vote being reversed. “The British have decided to leave. It is like cricket – when the umpire tells you you are out, you are out,” he said. “It is curious how effective that metaphor is.”