Film-maker Ken Loach joined union leaders and actor Ricky Tomlinson today to step up the pressure on the government to release documents relating to the case of 24 building workers arrested four decades ago after taking part in a strike.

Loach said the case, which saw Tomlinson imprisoned in 1972, was one of the "great causes of our time".

"It is an absolutely clear case so see let's see the strength of the trade union movement mobilised in the way it can be mobilised," he told a packed meeting in Westminster on Wednesday.

Earlier this week it emerged that Chris Grayling, the justice secretary, had told surviving members of the "Shrewsbury 24" that documents relating to the case will be withheld for a further 10 years because of national security concerns. The ban will be reviewed again in 2021.

But Loach dismissed the government's position and said time was running out for the remaining pickets.

"This can't go on much longer, it has been going on for 40 years, but we must make this happen now because otherwise these men will go to there graves ... without the truth coming out."

The meeting was attended by union leaders and Labour MPs including Tom Watson, John McDonnell and Dennis Skinner.

Francis O'Grady, general secretary of the TUC, said the union movement was fully behind the campaign.

"If it was not so offensive it would be laughable," she said. "So Ricky Tomlinson one of Britain's best known and best loved actors in his spare time is public enemy number one? Frankly if Ricky is a threat then every decent working man and woman in this country is a threat too."

Tomlinson and his fellow pickets were arrested in 1972 during the first ever national building workers' strike, which lasted 12 weeks and led to improved safety conditions and a pay rise. But the union's picketing tactics enraged the construction industry and government.

Five months after the strike ended 24 people were arrested and charged with offences, including conspiracy to intimidate and affray. They were convicted at Shrewsbury crown court in 1973 and six were jailed. Tomlinson got two years and Des Warren three years.

Their case became a cause célèbre for the left and unions, which believe the builders were victims of a government plot to make an example of trade union activists who took part in successful picketing.

Last year a letter obtained under the Freedom of Information Act revealed there was discussion at the highest level of the Heath government over the decision to prosecute the men.

In the document, dated 25 January 1973, Sir Peter Rawlinson, the attorney general, told the home secretary, Robert Carr, that the strike had produced "instances of intimidation of varying degrees of seriousness" and that he had to decide whether the men should be prosecuted.

Rawlinson said that Treasury counsel, to whom the director of public prosecutions had referred the cases, "took the view that the prospects were very uncertain, and … I agreed with him that proceedings should not be instituted". Despite this assessment, three weeks later the 24 men were charged.

The Tory government had close links with the building industry and was always suspected of being under pressure from that quarter to act.

The government says the documents are being withheld under section 23 of the Freedom of Information Act, a section relating to national security. Earlier this week a Ministry of Justice spokesman said the lord chancellor had renewed the decision made by his predecessor in not releasing remaining papers.

"The majority of papers relating to this case are already available from the National Archives. Where necessary material can be held for longer than 30 years under the Public Records Act."

But today Loach accused the government of hiding behind national security.

"What security aspect can be involved in this case in dealing with pickets who were trying to support other guys to come out on strike – what is the security aspect? The only security aspect can be the dirty tricks brigade getting involved. Those are the names and the mechanisms that will be revealed and that is why they don't want it out."