Archer Daniels Midland introduced longer shifts while Karen Barker, a 30-year-old laboratory technician, was on maternity leave, according to her union, Manufacturing Science Finance.
Ms Barker, who worked for the company for 14 years, was allegedly sacked without notice for failing to turn up for a 6am shift on her first day back at the laboratory in Erith, Kent.
Roger Lyons, general secretary of the union, said that Ms Barker, the only woman in her department, had requested alternative working arrangements so that she could look after her four-month-old child, but had been refused. Mr Lyons said the union would be taking Ms Barker's case to an employment tribunal alleging unfair dismissal and sex discrimination.
"It is beyond belief that a massive multinational like ADM cannot accommodate the needs of a working mother. Sacking Karen just before Christmas suggests it is not a very family-friendly company," he said.
The union is calling on the Government to introduce legislation giving parents the rightto new working arrangements after having children.
Mr Lyons said that Ms Barker's case had strong similarities to that of Annette Cowley, for whom the union won compensation for sex discrimination against South African Airways earlier this year.
Peter Bates, a director of ADM, which processes oilseed and soya, would not comment on the case. His company's profits exceed $1bn (pounds 600m) a year and it has 205 plants around the world, including four in Britain.Reuse content