The need for urgent airport expansion by 2030 could be diminished by Brexit, according to figures in a new study predicting the number of UK air passengers could be around 25 million fewer than forecast by government – or more than the entire annual traffic of Stansted.

Although airports have argued that Britain’s future isolation from the European Union requires rapid investment in airport capacity, the analysis by economists from airline industry body Iata predicts UK air traffic will tail off in the next two years, having experienced four years of rapid growth before the EU referendum.

Iata’s 20-year Passenger Forecast says that a hard Brexit will exacerbate an imminent loss in demand and leave passenger traffic around 8-9% below that resulting from a soft Brexit – the “most benign but arguably least likely scenario”, according to Iata.

While traffic growth will slow, Iata said that even under a hard Brexit, its 20-year forecast is for a significant increase of 45% more passengers by 2035, and that its report supports the case for increased capacity in the UK.

But the report’s forecast of 257 million UK flyers would equate to a total of just over 290 million passengers, including those transferring from one flight to another, by 2030. The most recent Department for Transport aviation forecasts from 2013, used by the Airports Commission in reaching its conclusion that a new runway should be constructed in the UK by 2030, predicted an increase to 315 million passengers by 2030.

Under that projection, the five largest south east airports – Heathrow, Gatwick, Luton, Stansted and City – were forecast to be full by 2030.

This claim was repeated this week by the transport secretary Chris Grayling at the Commons transport select committee. However, under the lower demand scenario that the DfT considered, London’s combined airports would not be full until 2040.

Revised projections from Iata suggest traffic will be nearer the lower limit of the forecast if the UK goes for a hard Brexit, compounding the impact of a falling pound and increased travel costs from limited access to the EU aviation market. Iata’s analysts said that reduced air capacity to and from the EU would “be expected to increase directly the cost of air travel with the bloc”.

If ongoing membership of the European Common Aviation Area is forfeited, Iata’s report warns, the impact would be “frontloaded” and the costs of air travel to the UK would remain higher for decades, dampening demand.

However, the weaker overall demand will make little difference to the main contenders for a new runway in south-east England, with Heathrow having effectively reached capacity in 2011, and Gatwick’s subsequent rapid growth seeing it forecast to reach capacity by the next decade if not before.

• This article was amended on Wednesday 19 October to remove speech marks from the headline and add a comment from Iata clarifying that its 20-year traffic forecast supports the case for increased airport capacity.