From my office window in Parliament I look down on Westminster Bridge and the pavement outside, which is covered in flowers. It’s a painful reminder of how a normal Wednesday afternoon turned suddenly into a day of horror and loss for ordinary Londoners and for one extraordinary police officer.

I can imagine the frantic reaction inside government. The Cobra situation room, packed with senior ministers, police officers and security officials, poring over the intelligence reports.

I know what it must have been like because I sat in many such meetings. I remember the immense pressure that ministers are under to reassure the public that everything is being done to keep them safe.

That’s why I have some sympathy for Amber Rudd, who yesterday attacked internet companies, especially WhatsApp’s use of encryption technology. She’s a new Home Secretary, confronted with this situation for the first time. She’s right, of course, that internet companies have a vital role in helping the police and the security services. And I share her obvious irritation when the industry reacts slowly to matters of public concern, like the recent furore over the placement of adverts next to extremist content.

But that’s where my sympathy runs out. Because Rudd’s proposals to end encryption as we know it are undeliverable and are not going to make London any safer.

First, the police have confirmed that Khalid Masood acted alone. There is no indication that he had any accomplices or that he used encrypted services to plan his brutal attack. He wasn’t under surveillance. There had been no request to read his messages. Second, while encryption undoubtedly makes the authorities’ job harder, you can’t ban your way to a better solution.

Rudd criticised WhatsApp and companies such as Apple for offering “end-to-end” encryption. Encryption technology renders a message completely private and unreadable by anyone who might want to intercept it in transit, whether that’s a hacker, a police officer, or a foreign spy. The only people who can read the message are the sender and the recipient. Even the company that provides the service cannot pry into your messages.

While this is obviously reassuring for anyone who worries about their privacy online — opinion polls suggest that many people are — it undoubtedly poses a dilemma for the security services too. While the authorities can ask to see a suspect’s messages, encryption means that the company simply doesn’t have them.

Surely it can’t be right that there are “safe spaces” where terrorists can communicate in secret? I agree. Who wouldn’t? But the reality is more complicated. The only reason the police know that Khalid Masood sent a WhatsApp message minutes before last week’s attack is that they have his phone. That tells you something important. If you want to read a suspect’s messages, you have to get into their handset. The security services can do that, either by getting hold of the phone or hacking into it. It’s not easy, but it’s possible and perfectly legal.

But the Government isn’t content with this. It wants to go further and order the companies concerned to use weaker forms of encryption, or face being blocked in the UK.

We’ve been here before. Powers that could have been used to force companies to remove end-to-end encryption have been on the statute book since 2000, but have never been used. David Cameron railed against end-to-end encryption in a speech in January 2015. Nothing happened. Why?

First and foremost because we’re talking about a useful technology. Businesses rely on it to prevent cyber crime and to reassure customers that their data is secure. It’s used in everything from banking to online shopping.

David Davis was heavily critical of Cameron’s plans. Writing in 2015, before he took up his current Cabinet position, he said: “Such a move would have had devastating consequences for all financial transactions and online commerce, not to mention the security of all personal data. Its consequences for the City do not bear thinking about.”

The second reason is that a ban would be impossible to enforce unless we took the dramatic step of cutting the UK off from the rest of the internet. End-to-end encrypted messaging services aren’t limited to WhatsApp and iMessage. The technology is essentially free and used by thousands of companies around the world. The Government might try to bar one or two of the big players from the UK but people would simply access other identical services in other countries. That’s the nature of the internet.

The only way to put a stop to end-to-end encryption would be to erect a Chinese-style “great firewall”, cutting the UK off from the rest of the internet and creating a “national internet” instead. That would be grossly damaging to our economy and to our civil liberties. Yet that is the logical conclusion of Amber Rudd’s statements.

So the Government is once again raising expectations that cannot be satisfied. That’s not to say there isn’t a problem — there clearly is — but the solution is more subtle and less headline-grabbing than ministers would like.

Intelligence agencies have long been in an arms race with technology. As I saw for myself in government, our agencies are extraordinarily smart at finding new covert technological solutions to new challenges. The solution isn’t to try to turn the clock back; it’s to call on the ingenuity of the boffins at GCHQ.

'A ban would be impossible to enforce unless we took the dramatic step of cutting the UK off from the rest of the internet' Nick Clegg

Politicians often grasp for analogies from the physical world when talking about the internet. Amber Rudd spoke yesterday of the days when spies would steam open envelopes.

But the truth is there is no neat analogy for end-to-end encryption. There’s a postman but no post office. There is an envelope, but when you steam it open the message is a jumble of meaningless characters.

Amber Rudd will quickly realise that she’s not only on a collision course with the tech companies, but also with an invention that can’t be uninvented.

The safety of Londoners depends on her taking a more sensible approach.