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The government is assuming that French preparations for customs and regulatory checks have markedly decreased the anticipated trade disruption from a no-deal Brexit, the BBC has learned.

"Reasonable worst case scenarios" still anticipate long disruption to about half the freight crossing the Channel.

But the assumption, the basis for a lot of the government's no-deal planning, has been upgraded twice recently.

The scenarios were told to industry, but kept secret from the public.

Last year the government's original "reasonable worst case scenario" for no deal was that for three to six months, 75-87% of "flow" across the "short straits" would be interrupted and forced to join queues on motorways approaching the Channel Tunnel and ports.

That assessment assumed that French authorities would check every lorry coming into its country manually. As there had been no talks between authorities, UK officials had to use satellite photographs to estimate the potential for holding and processing facilities around French ports.

In April this secret assessment, shared with industry via non-disclosure agreements, was improved to 50-70% of freight stopped.

This was principally because of the new preparations made on the French side of the border. For example, the Eurotunnel facility for UK freight can check nine lorries at the same time and provides parking for 100 vehicles.

In the past few days, the possible disruption to trade flow in the "short straits" has been further downgraded to 40-60% of traffic.

Better than it was, but still meaning thousands of lorries stuck on motorways in Kent and around Calais, blocking and delaying crucial trade. Even at this improved rate of flow, the available spaces for emergency lorry parking on the M20 and at disused airfields, would fill up over a fortnight, better than the two or three days assumed last year.

It is understood that even if the flow rate was 80%, much higher than currently assumed, medicine suppliers say they would require access to emergency freight provision currently being sought by the government again.

In reply, a government spokesperson said: "We are pushing on with all preparations for a no-deal scenario. There are already well-developed plans in place to manage any traffic disruptions in Kent."