Plans to make strikes illegal unless at least half of union members take part in ballots will leave them "with about as much power as Oliver Twist", the leader of the TUC has warned.

The Conservatives want to set a minimum turnout threshold for union votes after the next general election.

TUC general secretary Frances O'Grady called this was "anti-democratic".

But the Conservatives said "militant" union leaders should not demand strikes without majority backing by members.

The party has called for strikes to be illegal unless at least 50% of balloted members of a union cast a vote, with this policy to be included in its general election manifesto

It says the law as it stands allows a minority of union members to take action which damages the economy.

'Inconsistent'

But delegates at the TUC's annual conference in Liverpool backed a motion to "stand up against this political bullying" and launch a "major positive campaign explaining the democratic importance of trade union rights".

Ms O'Grady said: "The Conservative Party is not just proposing a few more bureaucratic obstacles that will make life a bit more difficult for trade unions... they would effectively ban strikes by the back door."

She added of the 50% plan that it was a "threshold no other ballot in Britain is required to meet and that many would fail".

The UK turnout for May's European elections was 34.2%, while at last month's by-election for a police and crime commissioner for the West Midlands it fell to just 10.4%.

'Vital'

Ms O'Grady said of the Conservative proposal: "It is an irrational, inconsistent and anti-democratic test."

She also criticised plans to alter laws on picketing, saying: "Dictatorships and authoritarian regimes routinely suppress trade unions and lock up strikers.

"Everyone who cares about civil liberties should be worried."

She added: "Unions can ask the employer for more - but they have about as much power as Oliver Twist had when he asked for more."

Conservative Party chairman Grant Shapps said it was "vital" that Labour leader Ed Miliband "joins Conservative calls to help put an end to the ability of militant trades union leaders to demand strike action without the majority of their ordinary hardworking members".

For Labour, shadow Cabinet Office minister Michael Dugher called the Conservatives' plans "desperate stuff "by a party that had "given up any pretence of standing up for working people".

At the four-day TUC conference unions representing about six million members across the public and private sectors will debate issues including jobs, the economy, public services and a planned EU/US trade deal.

A series of motions will call for increased campaigning for better pay, including taking co-ordinated industrial action in October.

Among the guest speakers at the event will be Labour's shadow business secretary Chuka Umunna and Bank of England governor Mark Carney.

Mr Miliband will not address delegates but he will talk to union leaders at their dinner on Monday.