A no-deal Brexit could lead to up to six months of chaos on key cross-Channel routes, the latest UK Government assessment has warned.

Ferries between Dover and Calais and traffic using the Eurostar Channel Tunnel service could be disrupted until the end of September 2019.

The warning – contained in a letter sent by Matt Hancock, the UK Health Secretary, to the NHS and the pharmaceutical industry - came as the SNP tabled its amendment to the May Plan to avoid the “dire consequences” of a no-deal Brexit by calling for an extension to Article 50.

The amendment, co-signed by Plaid Cymru, also urged the Tory Government to respect the will of the Scottish Parliament and the Welsh Assembly, both of which have rejected the UK-EU Withdrawal Agreement.

Ian Blackford, the SNP leader at Westminster, said: “We were promised strong and stable but what we have is a government in crisis. This SNP amendment seeks to prevent the UK crashing out of the EU with dire consequences.”

In his letter to the NHS and the pharmaceutical industry, Mr Hancock explained that while the UK Government could not know precisely what each EU member state would do in regard to border checks, Whitehall’s planning assumptions had been revised so departments could prepare for the potential impacts that the imposition of “third country controls” by member states could have.

"These impacts are likely to be felt mostly on the short straits crossings into Dover and Folkestone, where the frequent and closed loop nature of these mean that both exports and imports would be affected,” said the Secretary of State.

"The revised cross-Government planning assumptions show that there will be significantly reduced access across the short straits, for up to six months.

"This is very much a worst-case scenario; however, as a responsible Government, we have a duty to plan for all scenarios," he added.

UK ministers are drawing up plans to fly in vital drugs and give priority to lorries carrying medical supplies at gridlocked ports.

Paul Carter, Kent Council's leader, called for emergency measures to stop lorries entering the county to avoid chaos on the roads.

"We now need far more input and information from national Government in how they are going to work with us," he insisted.

"There must be a national freight transport plan which, when necessary, can hold lorries back from coming into Kent in the first place should the need arise."

Mike Thompson, head of the trade body, the Association of the British Pharmaceutical Industry, claimed that stockpiling was not the answer.

"With just 16 weeks until UK leaves EU, we need Government to take immediate action to open up alternative supply routes between UK and Europe and tell companies so that they can make plans."

Niall Dickson, co-chair of the Brexit Health Alliance said the body remained “deeply concerned” about the prospect of no-deal.

He said: "Given mutual reliance on supply of medical products between UK and EU, we must now see what contingencies the EU is putting in place to protect the health of citizens across the UK and Europe.

“We also encourage the Department of Health to work closely with the Department for Transport to ensure safe and steady flow of medicines," he added.

Meanwhile, the Border Delivery Group, a Whitehall co-ordination group for Government departments that have an interest in border issues, held discussions with key stakeholders on Friday to discuss the Government's no-deal planning assumptions for the UK border.

Elsewhere, leaked Government papers suggested that Ireland could suffer more from a no-deal Brexit than Britain with a projected seven per cent drop in GDP compared to five per cent for the UK.