Facing ferocious competition from low-cost airlines in the US, United Airlines has launched its most restrictive offering yet for budget-minded travellers.

United says: “Basic Economy fares have some important, additional restrictions compared to our standard economy fares”.

Couples or families booking together must acknowledge at the point of purchase that seating together is not guaranteed. Instead, automated seat assignments are given at check-in, and the airline warns there is “no opportunity to change” the allocated place.

United says: “If there are people in your group who need to sit together, this fare may not be the best option for you.”

Other big airlines, notably Delta, have launched Basic Economy fares — but United’s goes further by cutting the carry-on baggage allowance.

Only one “personal item,” measuring 22x25x43cm, is allowed on board; the maximum volume is 24 litres, barely half the normal size permitted for other economy passengers. Basic Economy passengers can check in additional bags, starting at $25 (£20) for the first case.

Ben Schlappig, author of the One Mile At A Time blog, said: “United really is taking basic economy to a new low.

“With the emphasis airlines place on on-time departures, you really have to wonder what this will look like in practice. Usually gate agents are scrambling to close the door on-time, so where are they going to get the time to argue with customers (which will inevitably happen) and process payments for checked bags?”

United’s move mirrors the policies on Spirit, which says: “If you don't select a seat we'll assign random seats at check-in for free.” Spirit, based in Miami, is selling seats at Ryanair-like prices, such as $35 (£28) for the 800-mile flight from Chicago to Dallas.