Reserve military officers could be called up for a year of service as part of government plans for a no-deal Brexit.
It comes after the defence secretary said 3,500 troops were "held at readiness" to help if the UK crashes out of the EU.
The official call-up order was made in a ministerial statement on Thursday.
Labour, which is calling for a no-deal to be ruled out, said the move showed the prospect was a "serious threat".
Reservists, who have military training but also hold civilian jobs, will make up one in ten of the troops prepared to be deployed before the scheduled Brexit date of 29 March.
Under the Reserve Forces Act, Defence Secretary Gavin Williamson can call on the officers to be sent "anywhere in the world" if it is "necessary or desirable".
The act says this can be done "for the alleviation of distress or the preservation of life or property in time of disaster or apprehended disaster".
The order will come into effect on 10 February and last until 9 February next year.
Reserve forces will be "on standby to deliver a range of defence outputs" and help perform "contingency plans developed by other government departments", armed forces minister Mark Lancaster said in a ministerial statement.
The Ministry of Defence said it has not yet received any requests for troops to be used by other departments.
It added it "routinely undertakes prudent contingency planning" and the reservist scheme would have "no major impacts on other defence outputs".
But Labour MP Ian Murray, who sits on the government's foreign affairs committee, said calling up reservists "shows just how serious the threat of a no-deal Brexit is".
"It is staggering that soldiers are being put on standby because of the risk of a constitutional crisis of the Government's own making," he continued.
Liberal Democrat armed forces spokesman Jamie Stone said the Conservatives have "made such a mess of Brexit" that "even soldiers are being put on standby".
"It is outrageous. What kind of message to the country is this? It does nothing to alleviate the worry and uncertainty people feel," he added.