Scotland's biggest teaching union, the EIS, public sector union Unison, and the STUC joined children's charities and anti-poverty campaigners urging the Scottish Government to adopt plans for English schools.
Deputy Prime Minister Nick Clegg announced last week that P1 to P3 children south of the Border will all be entitled to free school meals from September 2014 in a £600 million move. The Scottish Government is expected to receive a £60m windfall as a result, but could spend the cash differently.
In a letter to Deputy First Minister Nicola Sturgeon and cabinet secretaries for finance, education and health, supporters of the move said it "creates a huge opportunity for the Scottish Government to now fulfil its ambition of providing a free healthy meal to all children in P1 to P3".
Other organisations signing the letter included Save the Children, Children 1st, One Parent Families Scotland and the Child Poverty Action Group.
They said the Scottish Government should act "as soon as possible to confirm that P1 to P3 children in Scottish primary schools will also now receive a healthy school lunch from September 2014".
The SNP pledged to provide free school meals for all P1 to P3 youngsters before coming to power in 2007.
The number of families qualifying for free meals has been extended but the promise never fully met.