'Triggering Article 50 to leave the EU confronts the left and the labour movement with four specific challenges', John Foster told the Communist Party's executive committee at the weekend.
The first, he said, was how to develop a broad campaign to secure the progressive gains to be made from exiting the EU, such as winning back the powers to rebuild industry through state aid, public ownership, a state investment bank, public procurement contracts and economic regulation.
'These powers could be bargained away by a Tory government in neoliberal trade and investment deals with the EU and other countries', Mr Foster warned.
Secondly, he called for the right-wing populism of the Tories and UKIP to be countered with an anti-racist internationalism of the left, which was incompatible with supporting a 'neoliberal Fortress Europe' with its barriers against people and goods from the Third World.
This would have to include a fair, non-racist immigration policy within the framework of collective bargaining in which
workers have a major say. Thirdly, the CP international secretary emphasised the need to work closely with 'progressive forces for change' across Europe and more widely.
Mr Foster welcomed support from trade unions and political parties in Britain, Germany, France, Portugal and other countries for a seminar on the future of Europe on July 1 at the Marx Memorial Library in London, hosted by the Institute for Employment Rights, the Rosa Luxemburg Foundation and other bodies.
The fourth challenge, he declared, was to replace the 'politics of grievance' of the Scottish National Party with the 'politics of class' of a resurgent Scottish labour movement.
'Communists have always insisted upon the right of the Scottish people and nation to determine their own future', Mr Foster pointed out. But he questioned whether SNP policies to retain Britain's currency and monarchy together with membership of the EU and NATO amounted to real independence.
'Only federalism outside the EU and a massive transfer of wealth from the City of London's financial sector to working people throughout Scotland, England and Wales would provide a Scottish government with the powers and resources to intervene decisively in an economy dominated by the capitalist monopolies', he argued.
The two-day CP executive committee meeting also agreed plans for a series of public meetings across Britain between April and June on a 'people's exit' from the EU, with speakers from communist and workers parties in Ireland, Portugal, Cyprus and other EU countries.