"I could not agree more".
His European Parliament motion calls for an "association agreement" to deal with the future UK-EU relationship.
The UK will come to "regret" the decision to leave the EU, European Commission president Jean-Claude Juncker has warned.
An agreement on customs will be needed: May refers to two proposals already put to the EU.
On Wednesday, Britain's finance minister Philip Hammond called on the European Union to agree to a "mutual recognition" pact for financial services with the bloc and Britain accepting each other's rules to avoid fragmenting markets.
Kate Hoey, an Ulster-born MP for London and one of the few Labour politicians still carrying the flag for euroscepticism in the party, has voiced similar sentiments, remarking that Remainer threats of civil disorder in the absence of customs-free trade between Northern Ireland and the Irish Republic are "insulting" and only serve to encourage "men of violence".
Britain, which played no formal role in drawing up the draft, objected to some of its contents, particularly a section on avoiding a hard Irish border which set out in detail proposals to keep Northern Ireland in a the EU's customs union.
He added: "For the moment, it is not unlikely that the negotiations will end up with a bad result, a bad result for the European Union but worse for the UK".
Speaking in Strasbourg this morning, ahead of a vote to formally adopt a set of negotiating guidelines for the next phase of Brexit, Juncker told MEPs that any future relationship must be durable and not require continual renegotiation.
The UK and the European Union are set to negotiate a deal later this month that will largely keep things as they are until 2020, but a number of companies are already concerned about the long-term impact on the industry that employs over 800,000 people and generates turnover of £77.5 billion.
"After Brexit there is a big discontinuity in all the trade and services, but we still have a large market and it's supposed to be an interesting opportunity for all the financial institutions", Katainen said.
The study by consultants Oliver Wyman said country level differences are pronounced, singling out the agri-food industry in the Republic of Ireland and the auto and manufacturing sector in Germany as particularly exposed. Of course, associate members typically do not have voting power and have to rely on soft influence.
But the draft European Union guidelines, which will be worked on by European Union diplomats to be approved by the bloc's 27 national leaders in late March, spell out clear limits to this approach.
Jyrki Katainen also gives us the European Commission's take on other pressing issues, including moves to dump non-reusable plastics from our daily lives and clean up our oceans; and the latest steps towards merging at least some aspects of defence and the military in Europe.
"We are very much concerned with the situation, also the findings the United Kingdom has so far", Valdis Dombrovskis, the Commission vice president overseeing the euro, said.