Bill Morris, general secretary of the Transport and General Workers' Union, told BBC News: "I am amazed that here we are - the 21st century, years of sacrifices - and the government are promoting a right to ask as a major breakthrough."
The government is to announce the measures as part of its plans for family friendly policies in the workplace.
Employers will still be able to refuse requests if they will have a detrimental affect on their businesses.
Trade and Industry Secretary Patricia Hewitt will announce a new task force to advise on how family-friendly policies can be brought in without harming business.
The government has promised a new legal right for working parents.
Unions' fear
They will be entitled to ask for flexible hours ranging from starting work late to going part-time.
Employers will have to consider such requests seriously but can refuse them.
Trade unions who campaigned for a much stronger right to go part-time fear the government's plans will make little difference.
Bill Morris, general secretary of the Transport and General Workers' Union, told BBC News: "I am amazed that here we are - the 21st century, years of sacrifices - and the government are promoting a right to ask as a major breakthrough."
He demanded a proper legal right for employees to work part time.
There would be circumstances where employers wanted to "say no", but the reason for such refusals needed to be tested at an industrial tribunal.
"What we have here is a watering down of proposals from the green paper into a situation where the burden of proof has been shifted from the employer to the employee," he complained.
Ending the struggle
But the trade and industry secretary defended the decision not to force employers to offer the flexible hours.
Ms Hewitt said the government and the trade unions wanted the same thing.
"We want people to be able to balance their responsibilities as parents and their responsibilities as employees instead of having a struggle between being a good parent and a good employee," she said.
She argued the changes were a "sensible compromise which means that flexibility will be on the agenda of business".
"Businesses vary and what will work in one business won't work in another," Ms Hewitt told BBC News. "So you want some give and take here."