Unions have reacted furiously to new legislation that would tighten rules on strike ballots and political donations.

Under the Trade Union Bill, a turnout of at least 50% of members will be needed to authorise action.

In key public services - such as health, education, fire, transport, border security and energy - there will be an additional hurdle that a strike must be endorsed by 40% of those entitled to vote.

The package would also ensure that union members had to actively 'opt in' to political levies - the proceeds of which are overwhelmingly paid into Labour coffers - and reduce restrictions on firms' use of agency staff.

Business Secretary Sajid Javid said the Government was delivering a 'key commitment' from the Tory manifesto.

'Trade unions have a constructive role to play in representing their members' interests but our one nation government will balance their rights with those of working people and business,' he said.

'These changes are being introduced so that strikes only happen when a clear majority of those entitled to vote have done so and all other possibilities have been explored.'

But TUC General Secretary Frances O'Grady said: 'This Bill is an unnecessary attack on workers' rights and civil liberties that will shift the balance of power in the workplace.

'Getting a pay rise or defending terms and conditions will become far harder for working people. Even when ballots meet the government's new thresholds, employers will soon be able to break strikes by bringing in agency workers.

'If ministers were really interested in improving workplace democracy they would commit to online balloting. However, they would rather silence protests against their cuts to children's centres, libraries and social care services.

'These new restrictions on facility time will make it more much difficult for trade unions to solve problems at work before they escalate into disputes.

'Making it a criminal offence for seven people to be on a picket line is a waste of police time and not something you would expect in a country with a proud tradition of liberty.'

GMB General Secretary Paul Kenny said: 'When workers jump through the draconian hurdles required for their vote for strike action to be lawful employers can ignore the will of their own workers.

'Workers will have to give an employer 14 days notice of strike action. This is more than enough time for employers to legally hire another workforce to break the strike.

'This blatant one sided approach is guaranteed to poison the relationship between workers and their managers. It will lead to even more trouble.'

Mr Kenny said the proposals for scrapping automatic enrolment to political levies were designed to damage Labour.

'It is clear that the Tory Party High Command intend to make the Labour Party bankrupt by cutting off the main source of funding that they have relied on since the 1930s,' he added.

'This is a completely one sided approach to party funding. There are no proposals to force companies to ballot shareholders or to place a cap on donations from wealthy people when funding the Tory Party.'

Dave Prentis, general secretary of public services union Unison, said: 'These unfair changes will make it much harder for nurses, teaching assistants, midwives and other public sector workers to ever strike for a pay rise or challenge the behaviour of bad employers.

'These spiteful proposals will deny millions of ordinary workers a voice at work. Strikes are rare and the decision to lose a day's pay is never an easy one - especially for public sector workers who have suffered many years of pay restraint.'

Chris Keates, general secretary of teachers' union the NASUWT, said the Bill was part of a wider government campaign to trample 'those who fight for the rights of working people'.

'The Trade Union Bill and all this other legislation have a chilling common theme, to restrict, stifle and silence opposition and attack our fundamental democratic rights and freedoms,' she said.

'Now is the time for all those who value the hard fought for democratic rights and freedoms to expose and oppose the Government's real agenda.'

However, employers' groups welcomed the changes.

CBI Deputy Director-General Katja Hall said: 'The introduction of thresholds is an important, but fair, step to ensure that strikes have the clear support of the workforce.

'Placing time limits on ballot mandates is an important measure to ensure industrial action is limited to the original dispute and not extended to other matters.

'We welcome the consultation on modernising picketing rules. Intimidation or harassment of individuals is never acceptable.'

Dr Adam Marshall, Executive Director of Policy and External Affairs at the British Chambers of Commerce (BCC), said: 'Individuals and businesses depend on our public services, so the right to strike must be exercised with the greatest restraint.

'Higher thresholds should apply when a strike puts people at risk or affects the ability of large numbers of our fellow citizens to go about their day-to-day business and earn a living. Businesses will see this as a sensible piece of legislation that carefully balances the rights of those wishing to withhold their labour, against the rights of those who rely on access to essential services.'

Consultations on the 40% strike ballot threshold for key public sectors, picketing rules, and use of agency workers will be open until September.