The public’s disenchantment with Westminster is now so severe senior Conservative figures fear the party’s membership has fallen to 100,000 for the first time in modern politics – a fraction of its three million heyday.

The dramatic decline – similar to a haemorrhage in numbers of Labour and Liberal Democrat activists in recent years – prompted warnings that political leaders needed to adopt radically new ways of engaging with voters to avoid becoming obsolete in the internet age.

The warnings came as the Commons Speaker, John Bercow, backed a fundamental overhaul of Parliament to help it to reconnect with voters.

Download the new Independent Premium app Sharing the full story, not just the headlines

He suggested that centuries of “arrogant” control by ministers needed to end and raised worries that the institution would become irrelevant to people’s lives unless it was able to respond quickly to mass communications.

Conservative headquarters has come under fire within the party for steadfastly refusing to disclose its latest membership numbers. Its reticence has raised suspicions that the total is embarrassingly low.

Douglas Carswell, an MP pressing for fundamental overhaul of the party’s operation, said he would not be surprised if membership was now “south of 100,000”. Paul Goodman, the editor of the ConservativeHome website, said his “guesstimate” was that membership was between 100,000 and 130,000. Numbers are thought to have halved since David Cameron became leader.

The scale of the problem facing the party is underlined by a drop of revenue from membership fees of more than a quarter between 2010 and 2012. Numbers of signed-up members in some seats the party needs to hold at the next general election have fallen to fewer than 150 and, according to party sources, membership in other constituencies has “hollowed out”.

The drop means the Tories will struggle to find enough activists to knock on doors and post leaflets at the next election in the seats that hold the key to victory in 2015. They have recruited Jim Messina, Barack Obama’s campaign manager, to use social media to target crucial swing voters.

In the early 1950s three million Britons were paid-up Conservatives and the number remained over one million for much of the 1980s. When David Cameron became leader in 2005, membership stood at 258,000 and has fallen steadily since. A Tory spokesman refused to comment on its membership figures.

Mr Goodman said: “If the party won’t publish the figure, it surely suggests a problem and the only problem can be falling membership.”

Inside Politics newsletter The latest news on Brexit, politics and beyond direct to your inbox Enter your email address Continue Continue Please enter an email address Email address is invalid Fill out this field Email address is invalid Email already exists. Log in to update your newsletter preferences Register with your social account or click here to log in I would like to receive updates on politics every morning by email Update newsletter preferences

The former deputy Tory chairman, Lord Ashcroft, said: “We still have not received the membership numbers for the Conservative Party. Sadly I suspect it’s really bad.”

Conservative MPs say the rise of the UK Independence Party as well as some activists’ antipathy to same-sex marriage have contributed to the recent fall. The party, in common with Labour and the Liberal Democrats, is facing a demographic problem, with elderly supporters dying in greater numbers than the rate at which young members can be recruited.

Mr Carswell said the rise of social media “raises existential questions for political parties” run on a 100-year-old model. He told The Independent: “If you listen to people who whinge and whine and say the public aren’t interested in politics any more, that’s like saying people aren’t interested in music any more because they aren’t shopping at HMV. That would be ridiculous.”

Mr Carswell called for a drive to recruit registered supporters and internet members who would be given a say in the election of key Tory figures, including the party leader and chairman, and over policy.

“We face the stark choice of being a party that is unable to govern controlled by the whips and the whips’ toadies or a broad-based coalition capable of governing even if it does so in a noisy, chaotic way,” he said.

Michael Fabricant, a Conservative vice-chairman, said: “Membership of parties across the political spectrum, like the membership of clubs and associations, is declining as other attractions fill people’s time. That is why the Conservatives are introducing ‘team 2015’ to recruit people to help at the election. We are also looking at alternative forms of campaigning, which has led to the recruitment of Jim Messina.”

Labour’s membership peaked in the early 1950s, around the same time as the Tories’ high-point. In the election of 1951 almost 97 per cent of votes were cast for candidates of the two main parties.

Today the number of Labour members has fallen to fewer than 200,000, although there are signs it has been broadly steady since Ed Miliband became leader.

Mr Miliband may get a significant fillip, however, under moves to overhaul the party’s relationship with the trade unions. He wants to change the system to enable trade unionists to “opt in” to Labour membership. If only 5 per cent of people in unions affiliated to the party decide to do so, then Labour’s membership could jump to around 350,000.

Ukip said its membership stood at more than 30,000 compared with 19,000 last year.