The question of tactics on the issue of EU membership is tremendously important, because the theoretical justification of the L5I leadership contains arguments with consequences extending far beyond the EU question.
The arguments of the L5I leadership to vote for Britain to remain within the EU – the alleged advantages for the development of productive forces and for the international consciousness of the working class – are, of course, arguments calling for other non-EU European countries to join the EU (e.g., in Norway, Switzerland, Iceland, Moldova, Belarus, etc.).
And why, if so, should the new method of the L5I leadership be limited only to Europe? Such limitations cannot be justified by even one logical argument. It follows, therefore, that the L5I comrades could see themselves justified in agitating for Mexico to remain part of NAFTA, dominated by US imperialism. Furthermore, as a natural extension of this right-opportunistic logic, the L5I leadership could argue for the extension of NAFTA to other Latin American countries. Certainly, this would result in more favorable conditions for the development of productive forces and of the international consciousness of the working class, à la EU. If the L5I in Europe justifies expanding the EU with such arguments, why not apply the same method to other continents?!
The same opportunistic logic would then also lead the L5I to support the various free trade agreements between the EU and the US (TTIP), between the EU and Canada (CETA), between the US and several Asian and Latin American countries (TPP), or between China and a number of Asian countries (RCEP) – of course extremely "critical" support and naturally in conjunction with their call for "international class struggle."  According to their logic, such free trade agreements would "objectively" promote closer international integration of national economies and the working class! These are extremely concrete and important questions, as these free trade agreements are currently under negotiation, and MPs from the workers' movement must take a position on them. If the L5I leadership rejects these free trade agreements, then it would have to explain why in one case it is advocating membership in an international political and economic organization but in another case not.
Likewise, the L5I leadership would have to reject the exit of countries from the World Trade Organization (WTO), and would have to advocate joining that imperialist tool of oppression. This question would also be quite concrete, particularly when the growing rivalry between the US and China threatens to tear it apart.
All these examples show that the new position of the L5I on the EU and its justification inevitably drive them in the direction of social-imperialism. Despite their anti-imperialist rhetoric, they would support the concrete central projects of the EU and other imperialist powers – in the name of the "development of productive forces and of the international consciousness of the working class." Ultimately, the group would degenerate to becoming "critical" (of course) cheerleaders for the imperialist powers and their expansionism. What a sad end for a group that once embodied a proud revolutionary tradition!
It is very likely that the L5I leadership – frightened by the consequences of their right-wing turn based on opportunistic calculations – will indignantly reject calling to vote for the free trade agreements or WTO membership. But, if so, it will not be possible for them to explain why they use double standards regarding the EU and other such economic agreements.
Finally, Marxists know – and even the L5I leadership should not have forgotten this – the principle of the Prussian military theorist von Clausewitz, often cited by Lenin, according to which the "war is nothing but the continuation of politics by other means." If the alleged advantages of larger imperialist countries and business associations for the development of productive forces and of the international consciousness of the working class are actually so important for the L5I leadership, so much so that they are in favor of EU membership – then why not support achieving such greater political and economic state organizations by military means? Of course, the comrades will reject this as an "outrageous insinuation," and we do not for a moment doubt in the least their honorable intentions. But that does not change the objective logic of their position by means of which they unfurl the presumed advantages for the development of productive forces and the international consciousness of the working class and wave them above the political significance of the "tactical" support to imperialist states and confederations. Anyone who extends even a little finger to the program of social imperialism is inevitably caught in the net of its political chasms.
 On these free trade agreementzs see e.g. RCIT: Advancing Counterrevolution and Acceleration of Class Contradictions Mark the Opening of a New Political Phase. Theses on the World Situation, the Perspectives for Class Struggle and the Tasks of Revolutionaries (January 2016), chapter IV.1, http://www.thecommunists.net/theory/world-perspectives-2016/part5/