The National Trades Union Congress (NTUC) took aim at firms which had allegedly exploited Singapore's separation from Malaysia to retrench workers, in September 1965.
NTUC chairman Ho See Beng said it appeared that some of these firms were trying to take advantage of their staff on a misconception that with the separation, the Singapore Government was "economically weak".
"In Singapore, an employer would have to make a cast-iron case for retrenchment. Otherwise, he would be put in the 'dock' and exposed as an enemy of labour and an exploiter," said Mr Ho.
He told the media he lodged complaints with Finance Minister Lim Kim San against several firms that were planning to retrench staff.
Mr Lim said he hoped this would be settled amicably, adding: "I don't think it would be fair for anyone to exploit the present situation in the State."
Mr Ho said more than 200 workers had lost their jobs, with more likely to suffer the same fate.
He declined to name the companies involved, but a company in the news then for laying off workers was Danish firm East Asiatic Company, an importer of goods such as Carlsberg beer and Kelvinator refrigerators. It had reportedly planned to lay off 190 workers.
But the company's manager, Mr J.A. Hansen, called the figure "grossly exaggerated", though it was true that 85 mechanics, fitters and clerical staff would lose their jobs at the end of September as the firm was giving up its Fiat motor vehicle agency.
A big department store was planning to retrench 20 staff, while a firm that imports drugs and television and radio sets was set to lay off 30 workers, The Straits Times reported.
Mr Ho, while noting that Singapore trade unions operated in a "positive and enlightened manner", warned that firms which could not justify their retrenchments would face the wrath of NTUC.
"If employers revert to the days of the jungle, then we are prepared to oblige them and do the same," he said.
Meanwhile, about 500 workers of Fraser and Neave, who were members of the Singapore Manual and Mercantile Workers Union, went on strike on Sept 16, 1965, to protest against alleged discrimination against union members. The strike was called off after six hours after Labour Minister Jek Yeun Thong intervened.
