The opposition seized on the release of Operation Yellowhammer assessments of the impact of leaving the EU without an agreement to insist MPs return to Westminster.
It comes after Scottish judges branded the suspension of parliament “unlawful”.
While releasing analysis on impacts of no deal, the government refused to comply with a similar Commons demand to make public personal messages from special advisers regarding the controversial five week prorogation of parliament.
The move came as judgment was due on Thursday in a legal challenge that argued the government’s Brexit strategy will damage the Northern Ireland peace process.
The “reasonable worst case planning assessments” of a no-deal exit which were released at the demand of MPs showed that major hold-ups at channel ports could occur, along with “significant” electricity price rises and a return to a hard border in Northern Ireland.
On food, the document warned that some fresh supplies will decrease and that “critical dependencies for the food chain” such as key ingredients “may be in shorter supply”.
It said these factors would not lead to overall food shortages, “but will reduce the availability and choice of products and will increase price, which could impact vulnerable groups”.
“Low-income groups will be disproportionately affected by any price rises in food and fuel,” the document also said.
The analysis said the flow of cross-Channel goods could be reduced to 40 per cent of current rates on day one, with “significant disruption lasting up to six months”.
“Unmitigated, this will have an impact on the supply of medicines and medical supplies,” it said.
“The reliance of medicines and medical products’ supply chains on the short straits crossing make them particularly vulnerable to severe extended delays.”
As well, the document said “significant electricity (price) increases for consumers” were likely.
The release came after the Court of Session in Edinburgh found ministers had stopped MPs from sitting for the “improper purpose of stymieing parliament”.
It said advice given by ministers to the Queen which led to the five-week prorogation was therefore “unlawful and is thus null and of no effect”.
The government immediately announced it was lodging an appeal against the ruling with the Supreme Court, with a hearing set for next Tuesday.
The document’s assumptions are “as of August 2” this year, and it notes that day one after the scheduled EU exit on October 31 is a Friday, “which may not be to our advantage” and may coincide with the end of the October half-term school holidays.
“Protests and counter-protests will take place across the UK and may absorb significant amounts of police resource,” it said.
“There may also be a rise in public disorder and community tensions.”
The dossier said that on day one of a no-deal Brexit “between 50-85 per cent of heavy goods vehicles travelling via the short Channel Straits may not be ready for French customs.
It said this could reduce the flow rate to 40-60 per cent of current levels within one day, with unready trucks filling the ports and blocking flow, and predicting the disruption could last up to three months before lifting to around 50-70 per cent.
The document is very similar to one leaked last month, which the government insisted was out of date.
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