In my time, I have shared a platform with Theresa May at a Liberty event and Nick Clegg at the Lib Dem conference and on both occasions I remember being heartened by their criticism of the authoritarian policies of the Blair government. Here were politicians who knew the limits of state intrusion and seemed to understand the basis of British liberal democracy.

Four years later, May, as home secretary, is now advocating a system of total digital surveillance, while the deputy prime minister seems prepared to back the Communications Data Bill, provided a few meaningless safeguards are put in place. I won't say too much about May's position or Clegg's rush to compromise liberal principles, because this inconsistency is what we expect from politicians. It is the norm and I feel foolish for expecting otherwise.

The draft bill proposes to give the intelligence services and police total access to the nation's communications data, including text messages, phone calls, emails and internet connections. It is an extremely authoritarian power to be contemplated by a democratic society and it is almost exactly the same bill as the interception modernisation programme proposed by Labour.

The only difference between Labour's IMP and the new coalition bill is that, instead of building a central silo of information, the government is going to make phone companies, social network sites and internet service providers store and hand over your information. The police and intelligence services already have powers to gather communications data. More than 500,000 requests were made in 2010 and the number of requests has been steadily rising since the Regulation of Investigatory Powers Act came into force in 2000. The police and agencies have made good use of Ripa and nobody denies the value of such interceptions in fighting crime and terror. But this law will create a search dragnet that will catch vast numbers of blameless and unsuspecting citizens. Guilt will be suggested by association and by accident.

The crucial undemocratic element is that the monitoring will be done without the need for the police or agencies to apply for a warrant. No one will know the extent of the monitoring, its effects, nor the conclusions that the authorities may draw from the data. May's bill is, by definition, disproportionate and self-evidently breaches the Human Rights Act's guarantees on the right to a private life.

How these searches will work is not clear, but it seems likely that deep packet inspection boxes will be used to make successive refined searches on a person's communications patterns, linking them with other databases.

May suggests that an easy collaboration will spring up with such companies as Facebook and Google, but does that mean the UK's intelligence services will build a "pipe" into Google? The company is no angel when it comes to privacy, yet it may draw the line at becoming the official partner of state snooping, which is why it now defies Chinese authorities.

Labour's IMP system was priced at just over £2bn, which was criticised by the Tory and Liberal opposition as prohibitive. That analysis is even more compelling today. While we brace ourselves for the worst of the spending cuts, which are yet to come, the home secretary, police and intelligence services propose, over 10 years, to blow £2.5bn of our money to invade our privacy, at the same time providing the necessary framework for a totally monitored society.

Over the next few months, the government will come at you with arguments about terrorists and paedophile rings, as May cynically did, and it will tell you that you have nothing to fear if you've done nothing wrong, but the fact is that this government is going to give itself powers to spy on anyone for any reason it likes. There is nothing to stop the police and MI5 using these powers to gain information on legitimate activists and those at odds with the policies of the day.

It's a while since I have been using that kind of language, but the onerous truth is that the price of liberty is eternal vigilance and I am sorry to report that the same forces that persuaded Labour of the need to create a surveillance state have now captured the home secretary and are pressing their case among the more gullible members of the coalition.

We should not let this bill pass.

One good reason, as the Cambridge academic and database expert Professor Ross Anderson points out, is that old trick from the Labour years, which was to allow the secretary of state to make additions to the bill after the fact. This means that we have no idea what the legislation will eventually look like and it allows the secretary of state enormous discretion when it comes to the use of statutory instruments and afterthoughts of an oppressive and invasive nature.

We all have a lot to think about these days and there are many anxieties about the future, but we have to win this vitally important battle in order to preserve our democratic way of doing things, the respect for privacy and a person's right to keep something of themselves unobserved by the state.