Williamson told MPs on Tuesday afternoon that 3,500 soldiers would be "at readiness" in the event of a no-deal Brexit and would be deployed as requested by various government departments.
Williamson said the defence department would "have 3,500 service personnel held at readiness in terms of regulars and reserves in order to support any government department on any contingencies they may need."
Soldiers could be deployed at British borders to try to help manage inbound goods and people, as well as in the event of civil unrest.
Government ramps up emergency no-deal planning
The news comes after Downing Street announced that it would trigger all of its remaining emergency no-deal Brexit plans.
Ministers agreed on Tuesday to allocate an extra 2 billion in funding to try and manage the chaos of leaving the European Union without a deal.
A Downing Street spokesperson confirmed on Tuesday that Cabinet had agreed to dramatically ramp up preparations for a no-deal scenario.
Government departments will now activate all contingency plans drawn up by civil servants, including a move to book space on ferries to ensure medical supplies arriving in the UK do not run out.
The Department for Exiting the EU will also end out 80,000 emails to UK business groups and roll out a nationwide advertising campaign warning firms to prepare for a no-deal Brexit.
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