The Educational Institute of Scotland (EIS) teaching union said all children in the first three years of primary school should be entitled to a free school lunch from next year.
The call follows pressure on First Minister Alex Salmond to deliver on pledges made by the SNP when it first came to power in 2007.
A number of successful pilots were held to trial free school meals, but councils were given flexibility on how they rolled out the commitment and to date none has implemented it in full.
Pressure has also increased since the Treasury's Autumn Statement, which revealed the Scottish Government is to receive £308 million in extra resources - partly because all pupils in the first three years of primary school in England are to get a free school lunch from September 2014.
Larry Flanagan, EIS general secretary, said: "Ensuring all pupils in P1 to P3 receive a free school meal would be a significant step. It would aid family budgets, bring important health benefits for young children and help these pupils concentrate on their learning throughout the school day. We call on the Scottish Government to make a firm commitment to deliver this free meal pledge."
The Scottish Government said the number of eligible children registered for free school meals had increased by 19,000 since 2007.
A spokesman said: "We are committed to expanding free school meal provision further, but have been focusing on maintaining eligibility to free school meals under the UK Government's programme for welfare reform.
"Recently we were informed of the Barnett consequential arising from the UK Government's Statement . As the funding will form part of the Scottish Block Grant as a whole, we are giving careful consideration to how money may be used to the optimum benefit for children and young people.
"We fully recognise the importance of tackling poverty, including child poverty, and we are taking a long-term approach through our Child Poverty Strategy for Scotland."
Last week, children's charities, anti-poverty campaigners and the Church of Scotland appealed to Mr Salmond to press ahead with the policy. The organisations, including charities Children in Scotland, Shelter Scotland and Children 1st and union bodies the STUC and Unison, called on the Scottish Government to "grasp the current opportunity".
The group said research suggested a "universal approach" had a positive impact on the take-up of healthy school lunches, children's readiness to learn and their attainment.
A Scottish Government study into the pilots published in 2008 found the main reasons for children taking free school meals were because parents wanted them to have a hot meal, that they know they would get a healthy lunch, that the children liked the food provided and because they were provided free.
The study concluded: "Universal healthy free school meals have the potential to impact on children's health, wellbeing, and educational performance in the medium to long term."
However, it also noted the extra costs of the trial, which varied from £1.79 per additional meal in Fife to £4.65 in the Borders.