Preparing for a no-deal Brexit is the "highest priority" of Britain's Ministry of Defence, the department's top civil servant has revealed in a letter to staff.

Sky News understands a number of no-deal war games will be held by the armed forces in the coming days, culminating in a cross-government exercise towards the end of February.

The military's contingency planning is designed to assist the government in case of any disruption - such as blockages with the flow of goods in and out of the country - if the UK leaves the European Union without a withdrawal agreement.

Other details of the defence contribution to what the Cabinet Office is calling Operation Yellowhammer include:

:: The need to identify and prepare around 3,000 soldiers who are set to comprise the majority of the 3,500 military personnel that will be available for no-deal duties, such as assisting with freight at ports and airports. Training is due to start next month.


:: An awareness that selected troops could include soldiers on readiness to respond to a military crisis overseas, potentially impacting on the UK's ability to deploy if they are needed for no-deal Brexit duties as well, one defence source said.

:: An acknowledgement that military infrastructure may be needed to store freight or to accommodate troops and other personnel if large numbers of people are surged around the country to help manage any disruption.

:: An understanding that a plan to move a total of more than 6,000 soldiers and their families back to the UK from Germany by September could face disruption, though the MoD said the army is currently sticking to its planned timetable for the pull-out.

Stephen Lovegrove, the permanent secretary at the MoD, wrote to staff before Christmas to alert them to the urgent need to be ready in case of a no-deal exit from the EU.

"As a department, we have already made significant progress with our no-deal preparations," he wrote in the note, dated 20 December. Sky News has been read extracts of the note.

"This includes both our internal planning to mitigate the impacts on defence itself as well as the wider support we have provided to the cross-government effort…

"However, moving into the New Year we will not only have to continue this hard work but treat it as our highest priority."

He instructed the armed forces and other parts of defence to take all steps to implement the remaining elements of their no-deal plans "in full and without delay".

"If necessary, this will need to include delaying or deprioritising areas of work or reallocating internal resources to better meet no deal demands," Mr Lovegrove wrote.

The letter did not specify which areas could be affected.

However, it is understood that hundreds of civil servants from the MoD and its other agencies could be called upon to support the Cabinet Office and local government in contingency efforts if a no deal triggers disruption.

This would mean taking people away from projects, such as the procurement of equipment. This would not immediately affect front line operations but could mean delays to the delivery of needed kit and services over time.

On the military front, the bulk of troops earmarked for Operational Yellowhammer look set to come from 3rd (United Kingdom) Division, with officers saying if that is the case then the most likely impact will be that some of these units may already be on a raised level of readiness as part of a new UK-led initiative called the Joint Expeditionary Force (JEF).

This is a high-readiness force of more than 10,000 military personnel from a total of nine countries - also including the Baltic states, Norway, the Netherlands, Denmark, Finland and Sweden - set up to respond to military crises, environmental disasters or any other sort of emergency situation anywhere in the world.

The JEF is due to conduct an exercise in the Baltic Sea in the summer. The UK's participation in this exercise will not be affected by any no-deal Brexit call-ups, the MoD said.

The requirement for a total of 3,500 servicemen and women to be on no-deal alert is expected to run from March to July, with the number shrinking to the hundreds from July to October, according to a defence source.

Some 5,000 service personnel earmarked to mobilise if required in the event of a terrorist attack - a task known as Operation Temperer - will not be included in any no-deal planning.

A spokesman for the MoD said: "We routinely undertake planning across defence to ensure we are prepared for a range of scenarios and Brexit contingency planning is no different.

"We are working across government to ensure our people, the defence estate, and our national and international commitments would be unaffected by a no-deal Brexit.

"There would be no impact on previously agreed activity, including overseas operations, exercises and UK contingency operations."

Sky News revealed at the weekend that military chiefs have already spent at least £23 million on stockpiling food, fuel, spare parts and ammunition at bases overseas and at home in case supply lines are disrupted by a no-deal Brexit.