The British authorities have issued a second national security alert after Brussels indicated a terror attack in the German capital could be imminent.

Boris Johnson, Britain’s Foreign Secretary, has been at the forefront of the joint action, which allows the United Kingdom to be notified of heightened concerns.

His comments came as the nation prepares for a fresh national day of mourning, according to the Sunday Times.

Details of what security officials were doing were disclosed in the Federation of German Chambers of Commerce and Industry (BDI) release of the Europol Europe Portal, which Mr Johnson is credited with spearheading.

It said that the EU terror threat was “currently high” and the ongoing crisis in Europe meant Britain should be on “the alert”.

The publication noted: “There is no credible threat to the UK and security has been reinforced throughout Europe.”

The report comes amid fresh fears that a plot to attack Britain’s former colony, Malaysia, has been uncovered.

Britain’s House of Commons Intelligence and Security Committee (ISC) warned in July that attempts to attack the UK were a possibility.

The same security group has said the remnants of Islamic State terrorist group in the Middle East continue to raise “significant domestic concerns”.

A report said that militants “remain active and prepared to carry out attacks” and it has “possible” foiled plots “through intelligence services”.

Mr Johnson urged UK authorities to take on board the advice of the Europol EU Portal.

He said: “We should all be alarmed when extremist violence occurs here, and this puts our best defences under the spotlight.

“For too long the greatest cause of alarm has been the unknown threat of an attack in the UK.

“This threat is now being identified and it is the situation that we need to be vigilant to.

“It is the most important defence we have against a danger we all share – it is the most effective preparation we can give our public.”

The website is run by a European intelligence agency which has never had to issue a security alert.

And the location of its first alert was not made public by a European security organisation, the Forum of European Security and Defence Agencies (EISDA).

The document included warnings of a march in Dusseldorf on Wednesday and an imminent plot to “attack public places and institutions in the national capital city of Paris, the French capital.”

The November 2015 suicide car bombing in London killed four people, including a police officer.

The attack was believed to have been in retaliation for Britain’s military involvement in Syria.

In May this year, the European security agency cited an attempted coup in Turkey last month as a possible “threat to the UK.”

Following this, a No 10 source said the UK authorities were “acting on these indications carefully” but it was too early to say what part they might play in any such plot.

The Muslim Council of Britain, which represents Muslim organisations around the country, has warned that extremists are stepping up their attacks.