LONDON — With around 100 days until Britain leaves the European Union and no sign of its Parliament agreeing to an exit deal, the British government said on Tuesday that it would ramp up contingency plans for a disorderly or chaotic departure, including measures to put 3,500 troops on standby.

The government says it still expects to secure an agreement on withdrawal, known as Brexit, which would allow for a 20-month transition period during which little would change.

But fear of a more brutal rupture that could clog ports, starve factories and disrupt supplies of food and medicines is growing as the March 29 deadline for departure creeps ever closer.

It did not escape notice that the government’s deliberations were being advertised more openly than in past months, when it discussed almost everything in secrecy. That led to speculation that Tuesday’s decision might be a ploy to persuade a truculent Parliament to endorse the unpopular exit deal struck by Prime Minister Theresa May, ahead of a critical vote on the plan now expected during the week of Jan. 14.