A crackdown on Islamist extremism will fail to stop terrorism and could even make the problem worse, former business secretary Vince Cable has warned.

The Lib Dem, who was responsible for higher education during his stint in the cabinet, claimed planned new powers would leave universities so risk averse they would allow only a "bland exchange" of politically correct views.

It comes as an unlikely coalition of campaigners was formed to fight the plans amid fears they will curtail free speech and criminalise legitimate protesters and pressure groups.

In a speech to the Institute of Advanced Legal Studies, Mr Cable said he had blocked attempts to impose restrictions on so-called extremists during his time in government.

He added: "It seems highly likely that university authorities in particular will be risk averse and will seek to avoid the danger of legal action from the authorities in respect of extremist speakers.

"They will then also have to demonstrate impartiality by banning non-Muslim speakers whose reputation is also controversial - ie extreme - for different reasons.

"Instead of intellectual challenge there will be a bland exchange of views which are inoffensive and politically correct.

"This will not stop terrorism or terrorist recruitment and may even make the problem worse by driving underground those who are regarded as extreme but are currently non-violent."

Mr Cable, who suggested religious sectarianism in the West of Scotland was a form of "nonviolent extremism'", warned that prevention measures "can easily degenerate into surveillance" and has implications for prisons, councils, schools and nurseries.

He added: "Common sense suggests that anything which encourages secretive, suspicious behaviour is more likely to help terrorism than hinder it."

David Cameron earlier this month vowed to ''aggressively'' pursue and disrupt extremists trying to spread the ''poisonous ideology'' of radical Islamism.

Launching the Government's counter-extremism strategy, the Prime Minister warned it was no time for ''kid gloves'' when faced with the threat to Britain's values of democracy, equality and tolerance.

But the Christian Institute and the National Secular Society are among the organisations that have joined forces to battle against the reforms.

Tory former shadow home secretary David Davis, human rights campaigner Peter Tatchell, Green MP Caroline Lucas and former West Midlands chief constable Lord Dear are also backing the Defend Free Speech campaign.

Campaign director Simon Calvert said: "Defend Free Speech believe innocent people will fall foul of this unnecessary and dangerous piece of legislation. It will criminalise those who hold unpopular, unfashionable or challenging views.

"This could include pro and anti-religious groups, trade unionists and environmental campaigners. Indeed we have already seen police urging teachers to report on parents who go to anti-fracking protests."

The group warns that the threshold for imposing extremism disruption orders (EDOs), which could prevent people using mobile phones or the internet, is too low.

Mr Tatchell said: "Extremism disruption orders are too sweeping and too open to abuse. They risk transforming lawful activities into criminal offences in a way that threatens freedom of expression.

"The definition of extremism is far too wide and the threshold for an EDO is way too low."