Almost four months after the widespread adulteration of beef products with horsemeat was revealed by the Food Safety Authority of Ireland, there are growing fears in the UK that the investigation to identify the full extent of the fraud is being shut down, the Guardian has learned.

Senior sources in enforcement and the food industry have accused the Irish authorities of being more concerned to protect the Irish beef industry than to expose all the links in the chain.

"There is deep frustration. There's a belief the FSAI must have known exactly what it was looking for, but the Irish end is in lockdown and there is not the full flow of information we'd expect. We have a sense of immense pressure to close it down," a senior figure in UK enforcement said.

A high-profile victim in the food industry, said: "It looks as though the authorities are not going to be able to identify and prosecute any major abattoir or processor that sold undeclared horse because of a wall of silence from the Irish."

The shadow secretary for environment and food, Mary Creagh, called for more clarity over the investigation: "The question now is what progress are both governments making to bring people to justice? Ours has gone silent. If consumers are ever to see justice both sides will have to work closely together rather than going back to business as usual."

The beef sector is one of Ireland's largest industries, worth nearly €2bn in 2012. It employs almost 100,000 farm families and 8,000 workers in processing.

The horsemeat scandal led to millions of burgers and ready meals being withdrawn from supermarket shelves around Europe, but enforcement agencies say that where supply chains cross jurisdictions, they are not getting enough information. Industry victims report that their own efforts to find out where their meat was coming from are being frustrated beyond the immediate suppliers with whom they had legal contracts.

The Irish government vigorously disputes this account of its activities. A spokesman for the Department of Agriculture, Food and the Marine (DAFM) said it had conducted a comprehensive investigation with its own special unit and the police and had passed information about traders and other intermediaries in the supply chain outside its jurisdiction to Europol and other EU states in a transparent manner. Some of the tensions between the two countries have been played out publicly in the Commons environment, food and rural affairs select committee, where MPs had a robust exchange at the end of April with the head of the FSAI, Professor Alan Reilly, over what the Irish authorities knew and when. They accused the Irish government of putting its beef industry before consumers. Reilly responded that far from hiding the problem, his authority had been the first to uncover it.

The UK environment secretary, Owen Paterson, told parliament at the beginning of the scandal that the Irish were acting on a tip-off, and that he had been told this by its agriculture minister, Simon Coveney. Coveney has subsequently said they were not. Creagh said: "We have two completely different versions of early events. At the very least it raises questions over the political handling of this case."

The investigation is further complicated by the delicacy of the political situation where criminal activity crosses the border between Northern Ireland and the Republic, a senior official said. The border area is notorious for smuggling. During the Troubles organised crime, involving the movement of live animals, drugs and arms across the border, was linked with fundraising for paramilitaries.

"No one is wanting to stir up paramilitary history, and the banking crisis in Ireland makes it doubly difficult politically to go for the beef industry. There's a sense that enforcement agencies have known what's going on with meat, but that it might be better to let this play out," an industry insider told the Guardian.

The Food Standards Agency said: "Our jurisdiction is only the UK. Our investigations into whether any fraudulent activity has taken place in the UK are ongoing. We are continuing to cooperate with other regulatory authorities across Europe."

The select committee was sceptical about Reilly's insistence that the Irish had not been acting on intelligence. Its Tory chair, Anne McIntosh, accused him of "playing the innocent" and a Labour member, Barry Gardiner, described to Reilly a note of a conversation between him and the head of the UK Food Standards Agency, Catherine Brown, in February in which Reilly acknowledged using tests for horsemeat that were unaccredited and could not therefore be used to bring prosecutions, and said that Coveney's agenda was to protect the Irish industry.

The Irish authority tests were in fact meant to be a warning shot to its beef industry to clean up its act, which it knew was dirty but did not want to prosecute, Gardiner maintained. "That's a fantastic theory, but it's not true," Reilly replied.

Since January the food industry has poured resources into trying to pin down where horse entered its beef supply. As they have mapped the Irish companies and UK traders involved, industry and politicians have been struck by how many have family or business connections through common directors now or in the past. Key industry players are dismayed at DAFM's report in March which concluded that several of the companies that sold them horse warrant no further investigation. The companies say they are innocent victims in the current fraud. Although DAFM is critical of some of the companies that supplied horse, it has decided that several others can get back to business where subsequent tests have been negative and it has found no evidence that they knowingly used horse.

The Commons select committee is keen to question some of the companies and their directors further. It has invited ABP chairman Larry Goodman, other executives who used to work for him and now run their own meat companies, and the head of processing company Greencore, Patrick Coveney, to give evidence.

Goodman's ABP group is at the heart of the scandal, having supplied beefburgers that were 29% horse to Tesco from its Silvercrest factory near the Irish border, and frozen mince that was 29% horse to Asda from its Dalepak plant in Yorkshire. Other products from ABP to supermarkets also tested positive for horse at low levels.

ABP has admitted that rogue managers at its Silvercrest plant failed to follow supermarket specifications but says that it has otherwise been an innocent victim. Its chief executive, Paul Finnerty, has already been questioned by MPs about Goodman's history. Gardiner put it to him that an Irish public inquiry in 1994 found his companies had faked records, made fraudulent claims for EC subsidies, commissioned bogus official stamps, cheated customs officials, and practised institutionalised tax evasion in the 1980s.

Gardiner also noted that Goodman had been found in the past to have worked through a secret network of linked businesses known as the Cork companies and asked if there was another secret network involved in the current saga. Finnerty replied that all ABP's business today was conducted through ABP companies.ABP said it was inappropriate to bring up events that happened 25 years ago. Both it and the other companies say they have no current connections. Greencore was drawn into the scandal when fresh beef bolognese sauce it supplied to Asda, made using meat from ABP, was found in Asda tests to be 5% horse. It is based in Ireland; its chief executive, Patrick Coveney, is the minister's brother. Industry sources have been troubled by potential conflicts of interest.

DAFM said: "Minister Coveney and the department absolutely reject any suggestion of a conflict of interest. The fact is the Irish authorities were the first to disclose this problem which turned out to be pan-European."

Greencore said the Asda tests were a mistake and its own tests had come back negative. Its relevant operations were in the UK and came under UK regulators, so there was no conflict of interest, it said.