London - British Prime Minister Theresa May has warned of growing protectionism in the global economy, slamming plane maker Boeing for its role in the US government proposal to impose a massive tariff that could cost thousands of jobs in Northern Ireland.Addressing a conference organised by the Bank of England, May mounted a strong defence of free trade and insisted that her government would push for free trade after Britain leaves the European Union in March 2019.Undermines partnership"I think there is a real challenge for us globally today because I think that here are aspects of protectionism creeping in around the world," she said. "I have said before I want the UK to be a global champion of free trade, because I think those of us who believe in free trade need to stand up, not just explain its wider benefits, but help to explain its benefits to individuals".Her defence of free trade comes a day after Jeremy Corbyn, the leader of the main Labour Party opposition, made the case for more state involvement in the British economy and after the US Commerce Department proposed a tariff of almost 220% on Bombardier's C series aircraft, which threatens more than 4 000 jobs in Northern Ireland.May said the actions of Boeing, which had sought US government action over what it saw as unfair competition, "is not the sort of behaviour we expect from a long-term partner" and that it "undermines that partnership".Without directly threatening the US aircraft maker, May pointed out that the British government has various dealings with Boeing, particularly in the field of defence.Trade dealsBoeing alleged that Bombardier received at least $3bn in subsidies from the governments of Britain, Canada and the province of Quebec, allowing it to sell planes to US-based Delta Air Lines for less than the cost of production. The Chicago-based aircraft maker asked the Commerce Department and the US International Trade Commission to investigate what it called "predatory pricing".The spat has raised concerns that Britain and the US could end up in a tit-for-tat trade tussle. That's particularly uncomfortable for a government that is negotiating Brexit and hoping to tie up a subsequent trade deal with the US. One of the main motivations of those backing Brexit is that it will untie Britain's hands in trade as the EU currently negotiates trade deals on behalf of its 28 member states.