Britons travelling to EU countries face major delays and questioning by border officials if there is a no-deal Brexit, a leaked official document reveals.

The UK would also suffer a “degradation of border security”, after losing access to a crucial EU database with details about suspected criminals, the Border Force agency warns.

There would be “disruption to commercial flight schedules, leading to flights arriving out of sequence”, its memo also says – triggering big queues at airports, as well as to board the Eurostar.

In the presentation, the key Home Office agency admits for the first time that it will be unable to distinguish between EU residents in the UK and new EU arrivals, said Sky News, which obtained it.

It says there is the possibility of “disruption”, “loss of data” and an “additional clandestine threat” emerging from less co-operation with EU countries.

The no-deal plan also assumes that UK citizens will no longer have the use of faster “e-gates” when travelling to the EU – even though EU citizens coming to the UK will be allowed to go through them.

And it sets out proposals to introduce customs controls and shut off blue customs channels for UK passengers arriving back from the EU.

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The revelations follow an earlier leak, also from Border Force, which predicted freight capacity at Dover could be reduced by up to 87 per cent if there is a crash-out Brexit.

Diane Abbott MP, Labour's shadow home secretary, said: “The Tories continue to treat the public with complete contempt by concealing the real impact that a no-deal Brexit would have on our borders and trade.

“It appears that these leaked documents only scratch the surface of what Britain could be facing. Who knows what other catastrophic assessments the government is hiding from us?”

Sky News said the full presentation carried the title “EU Exit assumptions - NO DEAL”, and was stamped “Official – Sensitive”.

It highlighted impacts on:

* Travellers – because EU countries “will impose full Schengen Border Controls on passengers”, so they would no longer be able to use e-gates and would face “intentions checking” about the reason for their journey.

* Safety and security – because Border Force would have to rely on inferior Interpol data about wanted individuals instead of the latest Schengen Information System (SIS II) database.

There would be “loss of data and access to a number of key systems” that UK officials “rely on”, including “some frontline operational capability”.

* Flight schedules – “contributing to queues building at major airports [and] significant outbound queues at St Pancras”, where the Eurostar departs London.