The tobacco and drugs industries are among those that have used this tactic to thwart people who claim to have been harmed by their products.
Lawyers say it is increasingly common for powerful companies to lobby the Legal Aid Board in the hope that it will cut off help for groups of plaintiffs.
Although the firms point out that they are legally entitled to make representations, plaintiffs' lawyers say it is tilting the balance against ordinary people. The lawyers also believe that, with ministers calling for cuts in legal aid, the board is 'willing to latch on to any excuse' to refuse assistance.
Concern came to a head recently following an application for legal aid by several hundred people who said their health had been harmed by smoking. The request was turned down after the tobacco industry made extensive representations to the board, saying, in effect, that the claims had no chance of success.
Robin Lewis, a solicitor who represented some of the plaintiffs, said: 'What is uncomfortable is the sight of these companies, which are profitable beyond dreams, objecting to an extremely vulnerable group of elderly and dying people having access to public funds.'
Lawyers have expressed similar anxieties over cases brought by people who say they have become addicted to tranquillisers. Last month, the board suspended legal aid for 1,700 plaintiffs. Their claims against a drug company will only go ahead if they overturn the ruling on appeal.
Again, the board's decision followed representations made by solicitors acting for the multinational company Roche Products. Millions of pounds have already been spent on the case.
Plaintiffs' solicitors say they are effectively having to fight their entire case before the board in order to get help for their clients. While multinationals can draw on deep pockets to pay their lawyers, plaintiffs often have limited resources. Nevertheless, plaintiffs do sometimes obtain legal aid despite opposition from powerful firms.
The London Docklands Development Corporation, for instance, failed to stop residents of the area getting help with their claim for compensation for the noise and dust they say they have had to put up with.
Steve Orchard, chief executive of the Legal Aid Board, strongly denied that his officers were told to turn down potentially expensive cases. 'I can categorically refute that,' he said.
Defendants had always had the right to make representations to the board whatever the case, he said. But 'more issues are looked at more deeply (in group actions) than with a straightforward injury case'.