Foreign Secretary Boris Johnson demanded to know why Stop the War coalition, the group previously chaired by Jeremy Corbyn, has not publicly condemned the bombing in Aleppo.
During a heated debate in Westminster, Johnson asked "where is the Stop the War coalition at the moment? Where are they?" in reference to the lack of protests surrounding the humanitarian tragedy in Aleppo, one of the most fiercely contested cities in Syria's ongoing civil war.
Johnson also called for UK citizens to demonstrate against Russia's bombing of the ancient metropolis.
Stop the War was set up in 2001 to condemn what it perceived as unjust wars. Mr Corbyn, now the Labour leader, sponsored the founding coalition and has since served as chairman of the organisation.
The group claims to support the rights of Palestinians but has been largely silent on the issue of Aleppo, even though the city is home to a huge Palestinian community.
Mr Corbyn was heckled at the Stop the War conference on Saturday, with members reportedly shouting "What about Aleppo!" and urging him to call for the removal of Syrian President Bashar al-Assad.
Mr Corbyn has also been criticised for his perceived reluctance to condemn Russia over its involvement in the conflict.