After Theresa May rejected EU proposals for Northern Ireland effectively to remain in the customs union, Mr Tusk warned that progress in Brexit talks could be put at risk if there was "backsliding" on principles agreed past year.
However, despite Brexit not actually happening until next year, manufacturers will have to make a decision sooner regarding their future in the UK.
The European Commission is expecting to hear from the United States in the coming days how the European Union can avoid USA import tariffs on steel and aluminum, Commission President Jean-Claude Juncker said on Tuesday.
The UK will leave the single market and things are going to be different in the future, but the UK will not take part in a regulatory race to the bottom at the expense of workers' rights or the environment.
Firms across the EU's 27 countries other than Britain will have to pay 31 billion pounds a year in tariff and non-tariff barriers if Britain leaves the bloc without a deal, the report by Oliver Wyman management consultants and law firm Clifford Chance said. Nowhere is this more evident than in the draft legal text on Northern Ireland and the Republic of Ireland, which this resolution supports.
Mrs Foster herself is all too familiar with the violence which the Irish republicans brought on the Province - despite a referendum confirming a sizeable public majority in favour of remaining in the United Kingdom in 1973.
An agreement on customs will be needed: May refers to two proposals already put to the EU.
"We have a £75m deficit with the European Union of your goods and we employ 1.5m Germans through cars, etc - and the City of London is very important to funding European Union businesses".
The Ulster Unionist MEP will, alongside his party leader Robin Swann, meet with the EU's chief Brexit negotiator Michel Barnier and its chief Brexit co-ordinator Guy Verhofstadt in Strasbourg tomorrow.
The motion, prepared by the European Parliament's Brexit Steering Group, stresses that the EU has binding common rules, common institutions and common supervisory, enforcement and adjudicatory mechanisms, to make it clear that even closely-aligned third countries with identical legislation can not enjoy similar rights, benefits or market access to those of EU member states.
"This resolution calls for no diminution of rights, including employment rights and social rights".
Key departure conditions will need to be approved unanimously by the EU nations and the European Parliament.