Glasgow flights to New York to be grounded this winter
It is believed to be the first time the year-round route has been reduced to summer only since it was launched 19 years ago.
The blow came as Air France confirmed to The Scotsman it was axing its Glasgow-Paris route from October, just a year after it was launched.
United's daily service will now end in October, leaving no direct flights between the cities until they resume next May, apart from three return trips by Jet2 in October and November.
United blamed the move on a reduction in demand.
However, it appears to have been an unplanned move since tickets were already on sale and some passengers had already booked flights.
A spokeswoman said: "United will suspend its Glasgow-New York/Newark service during the upcoming 2017-18 winter season, 29 October 2017 to 4 May 2018 inclusive (westbound), in response to a seasonal reduction in market demand."
"The service will resume effective 5 May 2018 (westbound).
"We will contact customers with bookings for flights during the service suspension to either offer them alternative travel plans or provide refunds.
"We apologise for any inconvenience caused."
Delta launched summer flights from Glasgow in May which are due to end in the autumn.
United said year-round flights between Edinburgh and New York/Newark, and summer flights between Edinburgh and Chicago, would continue.
Air France-KLM said the Glasgow-Paris route, which was launched in March last year, was being ended because of lack of demand.
It competes with EasyJet to Charles de Gaulle, Paris' main airport.
A spokesman said: "The Air France-KLM Group's policy is to adapt its flight offering in real time, in order to best meet demand and take into account the current economic context.
"As of 28 October, Air France will be adapting its flight offering on departure from Paris-Charles de Gaulle by suspending its route to Glasgow Airport."
He said Edinburgh-Paris flights would be maintained, while Aberdeen-Paris flights would increase from two to three a day.
Aviation consultant Saj Ahmad, of StrategicAero Research, said: "It seems United has struggled to generate enough demand for Newark on a regular basis and therefore the costs of keeping it in place are simply too high.
"Whether passengers prefer JFK [in New York] or instead want to travel via Heathrow, United doesn't have competition, so it's clear their strategy was flawed.
"Newark has always played second fiddle to JFK and it's a mystery why the latter was never chosen since it could always offset costs with economies of scale from its bigger network operating at JFK.
"For once, this is a situation of United's own doing and it cannot blame Gulf airlines for it either."