A Labour Government would oversee the biggest overhaul in trade union laws in thirty years by giving workers the right to mount so-called “sympathy strikes”.

Shadow Chancellor John McDonnell said the policy, which would unpick legislation introduced by the Thatcher Government, echoed the action of striking workers who took a stand against the fascist Pinochet regime in the 1970s.

However, the Conservatives hit out at the “ludicrous” move which they claimed would take the country in the wrong direction.

Secondary action, otherwise known as sympathy strikes, can occur when trade union members take industrial action in support of workers who are employed by a different organisation.

Conservative Governments in the 1980s clamped down on the practice amid concerns that unions had too much power.

Tony Blair accepted the curbs and Labour’s 1997 manifesto was firm in that there would be no repeal of the laws passed by his predecessors.

“We make it clear that there will be no return to flying pickets, secondary [sympathy] action, strikes with no ballots or the trade union law of the 1970s,” it stated.

The Labour Government's employment relations white paper from 1998 also made it explicit that the days of secondary action were “over”.

Labour leader Jeremy Corbyn and McDonnell, who are on the Left of the party, have since said they support the right to go on a sympathy strike, but no commitment was made in Labour’s last general election manifesto

However an award winning film, Nae Pasaran, has put the issue of secondary action back into the spotlight.

Staff at the Rolls-Royce factory in East Kilbride showed their support for the people of Chile - whose democracy had been overthrown by a military dictatorship - by refusing to carry out repairs of fighter aircraft used by Pinochet’s supporters.

McDonnell said yesterday: “That group of Scottish workers took a courageous and humane act against the horrors of the Pinochet years.

“Such collective action and acts of solidarity have been attacked and undermined by restrictive anti-union laws started in the 1980s under Thatcher’s Tory government.

“The next Labour government will ensure workers can revive the spirit of the Rolls-Royce workers in East Kilbride, when they struck a blow against the brutal Pinochet dictatorship.

“In this way, we will open the next chapter of democratic society by enhancing democracy at work.

He added: “Our programme of workplace reform will restore the balance between employer and worker, and it will do so by installing basic trade union rights in law again.

“We will introduce a new Ministry of Labour to give workers a voice in parliament, sectoral collective bargaining to raise wages and conditions, and repeal dangerous anti-trade union legislation.”

Asked specifically if Labour would repeal the restrictions on secondary strike action introduced in the 1980s, he said: “Yes.”

Labour is also committed to introducing electronic balloting for industrial action, which the unions believe would make it easier to trigger strikes.

Scottish Conservative MP John Lamont said: “This is a ludicrous suggestion from the Labour Party that yet again shows they want to take us back to the 1970s. The current laws around striking were introduced for a very good reason, and insure against irresponsible strike action. There is no need or demand to change the current laws, yet John McDonnell is continuing to pursue his hard left agenda.”