The Home Secretary Amber Rudd has demanded that technology companies offer 'no place for terrorists to hide' in the wake of the Westminster terror attack.

Ms Rudd said that she was “calling time on terrorists using social media as their platform” as it emerged she will hold an urgent summit with firms later this week.

It comes amid a row over WhatsApp, the messaging app terrorist Khalid Masood used the seconds before he launched his deadly rampage last Wednesday.

Investigators are understood to be unable to see what was communicated.

Internet giants like Google have also come under fire for hosting extremist videos and other material.

It took just 82 seconds for Masood to kill four people, including a police officer, in Westminster.

Police say that they believe that he was acting alone and that they may never uncover the motivation for the attack.

Thousands have visited Parliament Square to lay flowers or pay their respects and a sea of flowers has now built up in tribute to the four victims.

Last week No 10 said that Prime Minster Theresa May believed that technology companies “can and must” do more to tackle extremism.

Ms Rudd called on encrypted messaging services like WhatsApp to build ‘back doors’ into their systems so that the intelligence agencies can read suspected terrorists' messages.

Ms Rudd also warned Google and others that they were now publishing, as well as technology, companies and had to take responsibility for removing extreme material.

The Home Secretary left the door open to changing the law if necessary.

But she said she wanted the industry to volunteer for reform.

On encrypted messaging services, she told BBC One's Andrew Marr Show: "It is completely unacceptable, there should be no place for terrorists to hide.

"We need to make sure that organisations like WhatsApp, and there are plenty of others like that, don't provide a secret place for terrorists to communicate with each other.

"It used to be that people would steam-open envelopes or just listen in on phones when they wanted to find out what people were doing, legally, through warrantry.

"But on this situation we need to make sure that our intelligence services have the ability to get into situations like encrypted WhatsApp."

On Sky News's Sophy Ridge on Sunday, Ms Rudd said: "End-to-end encryption has a place, cyber security is really important and getting it wrong costs the economy and costs people money.

"So I support end-to-end encryption, it has its place to play.

"But we also need to have a system whereby when the police have an investigation, where the security services have put forward a warrant signed off by the Home Secretary, we can get that information when a terrorist is involved."

Her call was backed by Europol, the European Union's law enforcement agency.

Europol’s director Rob Wainwright told the BBC Sunday Politics programme: "There is no doubt that encryption, encrypted communications are becoming a more and more prominent feature in the way that terrorists communicate, more and more of a problem therefore, a real challenge for investigators.

"And at the heart of this is a stark inconsistency between the ability of the police to lawfully intercept telephone calls but not when those messages are exchanged by a social media messaging board for example.”

Yvette Cooper, the Labour MP and chair of the Commons Home Affairs Committee, called on Ms Rudd to take action.

She said that the previous Prime Minister David Cameron had "lots and lots" of similar meetings with technology companies and "went around these issues again and again".

She said that the Conservative Government now "have to act".

But former Metropolitan Police deputy assistant commissioner Lord Paddick branded Ms Rudd's proposals "draconian" and said they would not have saved lives in the attack in London.

The Liberal Democrat home affairs spokesman said: "These terrorists want to destroy our freedoms and undermine our democratic society.

"By implementing draconian laws that limit our civil liberties, we would be playing into their hands.”

"My understanding is there are ways security services could view the content of suspected terrorists' encrypted messages and establish who they are communicating with.”