Speaking in London on Monday morning, the Prime Minister said of the withdrawal from the European Union: "It was never going to be easy or straightforward, and the final stage was always going to be the toughest.
"But we have in view a deal that will work for the UK, and let no one be in any doubt – I am determined to deliver it."
However, if Theresa May cannot get it through parliament, it would raise the possibility of the UK crashing out of the EU in March 2019.
A no-deal outcome could result in large amounts of disruption at ports and airports, with food and fuel supplies affected.
Does Brexit matter to British defence while the UK is part of NATO? Will Brexit mean a 'European Army' is more likely?
We asked two defence experts for some of the answers...
We asked two defence experts for some of the answers around Brexit and defence
Major General (Ret'd) Chip Chapman, former Head of Counter Terrorism at the Ministry of Defence, outlined why it was necessary to prepare for a no-deal Brexit: "If it was just general duty manpower, substitution manpower for the police for example, in breakdown of law and order there would be no additional training requirement placed upon people.
"If you had to generate additional capabilities such as drivers or things like that, then there would be a training period required.
"That is why it would be sensible to make contingencies if the government think that there are going to be these scenarios where military aid might be required."