Workers staged a sit-in today at the site of a new biofuels plant which has been at the centre of a row for weeks after hundreds of employees said they had been "locked out" of the contract.
Hundreds of workers held a meeting outside the Saltend site near Hull and were told that talks at the conciliation service Acas had broken down last night.
They decided to stage a sit-in at the wheat ethanol plant, which is being built by several contractors for energy giant BP.
The GMB has launched a hardship fund and called a national meeting of shop stewards to discuss a deterioration in industrial relations in the industry as a result of the dispute.
Officials said the continuing row at Saltend was "reprehensible", claiming that employers did not care about the fate of the 430 workers involved.
They have been out of work for nearly a month after the project fell behind schedule, leading to claims that they have been locked out of the job.
The union has donated £100,000 to the hardship fund and has called a shop stewards' meeting for April 18, saying that industrial relations in the engineering construction industry have deteriorated.
A protest is also being planned outside BP's annual meeting in London on Thursday.
Les Dobbs, regional official of the GMB, said today: "The employers walked out of the Acas talks although the unions were prepared to continue talking to try to resolve this dispute.
"We will now press ahead with a protest outside BP's annual meeting."
GMB general secretary Paul Kenny said: "It is reprehensible that neither the contractors nor the site's owners, BP, seem to care about these 430 workers who have been locked out. GMB does care and will escalate the campaign for justice.
"If BP thought that this problem would quietly disappear, they lack any real knowledge about GMB and its members. BP has had ample opportunity to help resolve the dispute but they have chosen to ignore the injustice to these 430 workers.
"There are growing fears that this 'lock-out' is nothing more than a ruse to bring in a different, lower paid workforce."
Vivergo Fuels, speaking on behalf of the new site, said in a recent statement that it had terminated a contract with Redhall Engineering Solutions (RESL) last month following "significant performance issues".
It added: "This contract should have been fully completed in February and yet the construction is less than 70% complete to date. New contracts will need to be placed but at this time there are no contractor organisations identified.
"Therefore, no new employment opportunities are currently available and this will be a matter for any potential new contractor. In view of this, it is not possible to offer any transfer of employment for the workers concerned.
"It remains both the legal and contractual responsibility of RESL to deal with its workforce. As the contract with RESL has been terminated, our Saltend site is no longer the place of work for the RESL employees. Therefore they have not been locked out, they simply no longer work there.
"There are 20 other contracted companies currently employed in the delivery of our Saltend-based plant, with a combined workforce of over 500 people. These remaining contractors are returning to our site this week and their work is continuing.
"Our priority is to open our world-scale biofuels business as soon as possible, bringing long-term economic benefits and jobs to this area."
Vivergo Fuels said in a later statement: "We are appalled by the behaviour of some demonstrators including an element of the Redhall Engineering Solutions workforce earlier today, outside our Saltend site.
"We have heard reports of significant physical and verbal abuse directed at employees on the wider Saltend complex as well as the general public.
"This action is both irresponsible and ill-considered, with those involved being determined to cause the maximum disruption to parties completely unassociated with their dispute.
"We understand that organisers have staged this demonstration following our termination of the contract held with their employer, Redhall. Despite this being a matter between the affected workforce and their employer, Redhall, we are committed to working with the unions concerned in a bid to help resolve the issues raised.
"In order to bring new contractors in to complete the construction and in turn promote any new employment opportunities, it is imperative that access to our site is not prohibited or impacted by the ongoing protests. When the new contractors are in place any new employment opportunities will be advertised so that former Redhall workers can apply for the resulting jobs. This has been stressed to the unions.
"As part of our commitment to provide assistance in resolving any outstanding issues with the Redhall workforce, yesterday, Vivergo Fuels brokered a meeting with the unions concerned. In order to ensure that meaningful progress could be made, Vivergo asked that the unions involved instruct their members to stop picketing the Saltend site gates as this has also been causing disruption to other companies on and around the Saltend complex as well as to local people.
"The unions weren't prepared to do this and despite understanding our position from the ongoing discussions we've had with them over recent weeks, the meeting at Acas made no progress. For reasons known only to themselves, the unions do not appear to be in a position to discuss reasonable options to resolve this situation."