Leaving the EU with no deal would have “an impact on protecting the public”, according to the home secretary.
Opening the second day of debate over the PM’s Brexit withdrawal agreement, Sajid Javid backed her deal, saying it would ensure the UK stayed “one of the safest countries in the world”.
But he admitted there was “further work to be done” on future co-operation.
Shadow home secretary Diane Abbott said, at its worst, the deal would be “leading us off a cliff, on security”.
Mr Javid also revealed his intention to publish a White Paper in December detailing plans for a new immigration system – but he could not guarantee the date.
He had earlier told the Home Affairs Committee that he hoped it would be released before the meaningful vote on the Brexit deal next Tuesday.
But pushed further about its publication date, Mr Javid said: “This is the most significant change in our immigration system in 45 years.
“Rather than rush the White Paper, it is important that we focus on the detail and we get it right and we get it out as soon as possible.”
Ms Abbott said there had been promises the plan would be published several times since the summer of 2017, but it was still “missing”.
She added: “What confidence can anyone have in post-Brexit immigration policies when ministers still seem as though they don’t know what they want and they can’t agree on what they want?”
Although Wednesday’s eight-hour debating session is focusing on the two issues of security and immigration, former Tory minister Sam Gyimah took the opportunity to speak from the backbenches for the first time in six years.
Mr Gyimah – who quit his job as universities and science minister over Mrs May’s Brexit deal – told the Commons he “loved” his job and did not take the decision to step down lightly.
But, he said the deal was “not politically or practically deliverable”. He added: “Before we are Leavers or Remainers, we are British and it is the national interest that we care most about.
“The home secretary admitted that it might not be perfect, almost implying that this was like trying on a pair of shoes that were not the right colour and may be a little bit tight, but you could get on with it and it would be fine.
“But these are like shoes that have got holes in the sole. This deal is fatally flawed.”