As the only Green party peer I receive a lot of post to my office in the House of Lords. Rarely, though, do I open letters like the one that has been revealed. The anonymous writer alleged that there was a secretive unit within Scotland Yard that has used hackers to illegally access the emails of campaigners and journalists. It included a list of 10 people and the passwords to their email accounts.

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As soon as I read the first sentence of the letter, I knew the content would be astonishing – and when some aspects of the letter were corroborated by lawyers and those on the list – I was convinced that we owed it to this brave whistleblower to hold the police to account.

The list of allegations is lengthy. It includes illegal hacking of emails, using an Indian-based operation to do the dirty work, shredding documents and using sex as a tool of infiltration. And these revelations matter to all of us. None of us knows whether the police organised for our emails to be hacked, but all of us know the wide range of personal information that our emails contain. It might be medical conditions, family arguments, love lives or a whole range of drug- or alcohol-related misdemeanours.

When the police act with impunity, all of our private lives are put at risk. Whether you’re involved in a local campaign against library closures, a concerned citizen worried about air pollution or someone working for a charity – who’s to say that officers won’t be spying on the emails you send? The police put me on the domestic extremism database during the decade when I was on the Metropolitan Police Authority signing off their budgets and working closely with officers on the ground to fight crimes such as road crime and illegal trafficking. If someone in my position – no criminal record and on semi-friendly terms with the Met commissioner – can end up on the database, then you can too.

The truth is that without the bravery and professionalism of two serving police officers who have blown the whistle on state snooping I would know nothing about my files, and those of other campaigners, being shredded by the Domestic Extremism Unit. We would have had no suspicion that those files had been shredded to cover up the illegal hacking of personal and work e-mails by the police.

Please don’t fall for the old establishment lie that the problem is a few rotten apples. This alleged criminality is the result of a deliberate government policy of using the police and security services to suppress dissent and protest in order to protect company profits and the status quo. Such an approach inevitably leads to police officers overstepping the mark as they feel emboldened by those at the top levels of government and an immunity from prosecution provided by senior officers keen to please the people who decide their budgets.

The police don’t always act as neutral agents of the law. We know that the Thatcher government’s determination to break the miners’ strike led to the Orgreave confrontation in 1984. There are still allegations about the links between the police and those running blacklisting databases that led to hundreds of construction workers being condemned to unemployment and poverty.

And don’t mistake this for a partisan attack on Conservative politicians. Theresa May has forced through the draconian Investigatory Powers Act, but the Labour party too has been timid at best in opposing this snoopers’ charter. Indeed it was the Blair government that left a legacy of draconian public order laws, and which broadly defined the anti-terrorism legislation upon which an edifice of modern surveillance powers has been constructed.

Many are unaware that joining an anti-fracking group, or going on a demonstration, could get you labelled a domestic extremist, photographed, questioned and followed for months or even years – without ever having been convicted of a crime.

It’s only by speaking out against these intrusions that we are able to challenge this rotten culture of impunity. After all, it was David Cameron who gave us the Hillsborough inquiry and Theresa May who set up the Pitchford inquiry into undercover officers. Politicians don’t always do things for good reasons, but they do respond to public pressure.

Change is possible, but in the meantime, we should be doing everything we can to make it hard for the police to spy on us. Use encryption, two-step email security and other precautions suggested by organisations such as Liberty. Don’t stop saying what you think, or working to make the world a better place, but do assume that the police will be working to protect the companies, banks or energy companies that you want to challenge.

It isn’t how things should be, but the evidence shows that is the way things are.

A campaign to get the police out of the lives of environmentalists and social justice campaigners is a good start, but it will fail unless it reaches out – starting by working with those in the Muslim community intimidated by Prevent.

Above all, we must convince the middle ground of society that everyone will be safer if the security services focused on what we all want them to do – stopping terrorists and serious criminals. This is not unreasonable, and the starting point is a change to the legislation so that it narrows the definition of terrorism to exclude the nonviolent, noisy and rebellious