Following a meeting of her cabinet – the last before Parliament breaks for the Christmas/New Year break – Defense Secretary Gavin Williamson said around 3,500 troops would be on stand-by to help deal with the "any contingencies" from a no-deal outcome, AFP reported.
But Brexit Secretary Steve Barclay said ministers still hoped to secure backing for May's agreement in a House of Commons vote delayed until next month.
He added that any responsible government would intensify preparations for the "default option" of leaving the European Union without a deal on March 29 next year.
"We agreed that preparing for no deal will be an operational priority within government," Barclay said. "But our overall priority remains to secure a deal."
He said departments would step up their advice to businesses on how best to prepare for the scenario, which many fear could be catastrophic for the British economy.
Businesses will be provided with a 100-plus page online preparation pack and emails will be sent to 80,000 of those most likely to be impacted over the next few days, according to Downing Street.
May's cabinet is divided between ministers who want the government to embrace a no-deal outcome and those who favor Parliament having a final say in a series of votes on potential scenarios for Brexit.
One option is the prime minister's own Brexit deal, but that is hanging by a thread after she was forced to pull a vote on it last week in the face of huge opposition from within her own Conservative Party.
Earlier Tuesday, Downing Street reportedly blocked opposition leader Jeremy Corbyn's request Monday for a non-binding no-confidence motion in May.
That had followed her telling MPs they will not get to vote on her deal this week, but instead in the week beginning January 14.
The delay prompted accusations she was stalling to increase pressure on MPs to back the plan – and fresh calls for Labour to table a binding no-confidence vote against the whole government.
However, such a move faces likely defeat after Conservative Brexit hardliners and the Democratic Unionist Party, which props up her government, both said they would back May.
May insisted Monday that she was continuing to seek "assurances" from the EU over elements of her plan, although EU officials said no meetings were planned.
The postponed vote has left members of her own party and Labour opposition politicians infuriated.
"The prime minister has cynically run down the clock, trying to maneuver Parliament into a choice between two unacceptable outcomes" – her deal or no deal, Corbyn said.
May is also facing calls for a second referendum to resolve the impasse, with dozens of MPs from all sides now supporting another poll and reports that May's officials are also considering the possibility.
But the prime minister argued that this would betray the 2016 Brexit referendum result and undermine public confidence in politics.
"Let us not break faith with the British people by trying to stage another referendum," she told Parliament on Monday.
"Another vote... would do irreparable damage to the integrity of our politics," May said, adding that it would also "likely leave us no further forward".