The cessation comes despite attempts to rescue the route in a £9m emergency deal, mostly from Stormont.
Economy Minister Simon Hamilton had previously said the rescue package was crucial to attracted US-inward investment for Northern Ireland.
A United Airlines statement said: 'We have regretfully taken this decision because of the route's poor financial performance."
The ruling by the European Commission to block the funding package to protect Northern Ireland's one and only scheduled trans-Atlantic service defies logic and is an example of "abysmal Brussels decision-making".
Managing Director Graham Keddie said:  "You could hardly get a worse example of process-driven madness.
"This is a vital link for business and losing it will be a body blow to executive ministers who use it to promote Northern Ireland to would-be investors from the United States.
"This is a bad day for the executive and a bad day for Northern Ireland."
"The executive did the right thing with its bid to save this key route. There was a risk to the flight and we stepped in to save it. Faced with the same circumstances again, I would make the same decisions. All public money has been recouped with interest and we retained the route for a longer period.
"Our decision to support the route was praised by many and, had we not have made an effort to save the United flight, we would have been rightly criticised.
"We were always aware of the potential of an issue around state aid compliance, but given the tight timescale to put a package of support in place, approval by the European Commission could not be sought in advance of agreeing a deal with United."
"This is an international embarrassment.
"This is a huge blow to our international standing and the minister must tell us what he intends to do to sort it out."
"Awarding a £9m subsidy for a global corporation, which makes billions every year, against the advice of senior civil service officials and against state aid rules, was a dangerous commitment of resources at a time of already stretched budgets.
"The gamble taken by the executive has spectacularly crash landed and there are now very serious questions for the economy minister to answer.
"We need to know what communication the minister had with the European Commission before and after he issued a ministerial direction, ignoring the advice of senior civil servants.
"Additionally, Simon Hamilton must outline how much public money has been spent advertising the route since the direction was issued."
Alliance MLA Stewart Dickson said the decision by United Airlines to halt its Belfast to Newark route is a serious blow to Northern Ireland, in economic and social terms.
He said: "To lose our only direct air route from Northern Ireland to the United States will have a serious impact on our business community as well as local people using it to visit family or for holidays. Coming just weeks after United was offered £9 million to keep the route open after they threatened to pull it is a slap in the face to our Executive.
"We now learn that the funding was blocked by the EU Commission and I am calling on the Executive to explain what discussions were held with the Commission before the announcement, which was made with great fanfare. Is this another case of the Executive seeking good news headlines without adequate preparation? Did they even bother to check if this funding could break EU rules? It looks like the Executive and the Department for the Economy have let us down again.
"I have consistently challenged the Minister for Economy to reveal the evidence that he used in deciding to allocate this money to United Airlines in the first place. Every time he has been unable to give a satisfactory answer.
"Now that he has also been knocked back by the European Union, it is evident that he didn't even bother to check whether he was allowed to do it, let alone complete an economic appraisal for it.
"Maybe, in future, the minister will consider the need for proper due process to ensure that money is being spent on things that it can actually be spent on, and that will actually have a benefit to society as a whole."
"I was against this subsidy being given to United in the first place.
"There are better uses for the £9m, especially when we are always being told that money's too tight to mention for a lot of departments.
"There was never a guarantee. If the European Commission had not put a stop to it (the deal), there was no guarantee United was going to preserve the route anyway."
Politicians and businesses have been reacting to United Airlines ending their flights from Belfast to Newark Airport.