Turkey began a manhunt for the killer, who also injured nearly 70 people at the Reina club 80 minutes into the new year when he opened fire with an assault rifle.

The Site Institute, which monitors terrorist groups, said yesterday that Isis and its supporters had “mounted a massive campaign against Turkey” in recent days.

The Istanbul killings are thought to be revenge by Isis for Turkey’s alliance with Russia to destroy the jihadist network in Syria. The Isis leader, Abu Bakr al-Baghdadi, has urged his followers to “seek help in Allah and attack Turkey, turn their security into panic, and their calm into chaos”.

Security experts say that even as Isis loses territory in Iraq and Syria, its capacity to create mayhem in Europe is undiminished. Days ago an online message from a pro-Isis group called for attacks by “lone wolves” on “celebrations, gatherings and clubs”.

In Istanbul a French woman, a Canadian man and a Bollywood producer were identified among the 24 foreign dead as Turkey suffered its fourth terrorist attack in a month. It is not yet known whether any Britons were involved.

Theresa May wrote to President Erdogan of Turkey to offer condolences for the “further devastating attacks” and said that Britain stood “ready to help” with whatever was required, including intelligence-sharing.

Ben Wallace, the security minister, said that Isis was planning a chemical weapons attack in Britain. “Experts have warned that their ambition is a mass casualty attack and they have no moral barrier to using whatever means possible,” he said.

Mr Wallace also warned of the threat from “the enemy within” as terrorist groups, Russia and cyberattackers were trying to plant “traitors” in the government, the military and businesses. “There are traitors. We have to be on our guard,” he said. “The insider threat, as we would call it, is real and it can be exploited and there are people trying to do that as we speak.”

Closed-circuit television footage showed the killer as he opened fire outside the club and appeared to remove some of his clothes while surrounded by the dead on the dancefloor. Turkish special forces arrived on the scene within minutes, but the killer escaped and left his weapon behind.

Some of the dead are thought to have drowned after jumping into the freezing Bosphorus Strait, which runs alongside the club. Hours before the attack, Mr Erdogan had delivered his end-of-year message, speaking of the terrorist strikes that had hit the country. He hoped that 2017 would be more peaceful. After the killings, he said: “They are trying to create chaos, demoralise our people, and destabilise our country with abominable attacks which target civilians. We will never give ground to such dirty games.”

No organisation had claimed responsibility, but the target and method of the Istanbul attack, with its echoes of the Paris Bataclan nightclub shooting of November 2015, point to Isis.

If that is proved to be the case, the bloodshed will have brought one of the most exclusive parts of Istanbul into collision with the killing fields of northern Syria, 800 miles to the south. It will also polarise Turkish society, which is split over Mr Erdogan’s Syria campaign.

Mehmet Kocarslan, the owner of Reina, said that he had received warnings of a possible attack and had stepped up security. “The US intelligence warned over such an attack about one week or ten days ago and measures have been taken.” The US embassy has denied that it provided any intelligence.

• Instances of charities being reported to the security services over suspected links to extremists have almost tripled in the past three years. The Charity Commission raised concerns 630 times in 2015-16, the highest number it has recorded, compared with 234 three years ago. It investigated four allegations of abuse of charities for terrorism. William Shawcross, the commission chairman, said extremists taking over charities was its most deadly challenge.