A report by the Lords has warned the UK would be "deeply damaged" by a no-deal Brexit.
The Lords EU Committee found that withdrawing without an agreement would not only hit the economy, but impact counter-terrorism, nuclear safeguards and aviation.
It is "difficult, if not impossible, to envisage a worse outcome for the UK" than a no-deal scenario, the report said, adding the impact on trade would be "grave", and UK ports would be "overwhelmed" by customs checks.
The report also said the “overwhelming weight of evidence” suggests leaving the EU by March 2019 “will be impossible" and enshrining that date into UK law would “not be in the national interest."
Peers argue that the Government should extend UK membership of the EU for a limited period, or at least set a withdrawal date much later than March 2019 to ensure a secure legal basis for a transition period.
The report urged the Government to abandon its "nothing is agreed until everything is agreed" stance on talks because any last-minute failure to ratify a deal would result in a "chaotic Brexit," while concern was expressed for a "bare bones" deal, saying such a move would also be "very bad."
"The overwhelming weight of evidence suggests that 'no deal' would be the worst possible outcome for the UK, in terms of the economy, security, the environment and citizens' rights,” Acting committee chairman Lord Jay of Ewelme said.
"The biggest risk factor that might lead to a no-deal outcome is time - the clock is ticking.
"While we support David Davis' ambition to secure a comprehensive agreement by 29 March 2019, almost nobody outside the Government thinks this will be possible.
"The negotiations may need to continue beyond that point, and enshrining the deadline in domestic law would not be in the national interest.
"Both sides agree that we will need a transition period, to give confidence to businesses and potentially to buy time to complete the negotiations.
"But the Government hasn't yet explained how transition will work, or what its basis will be in EU law.
"The evidence is clear that in legal terms the most secure way to buy time is to use one of the options that exist within Article 50 for a time-limited extension of the UK's EU membership.
"If buying a bit more time means that we get a better outcome, which benefits businesses and citizens on both sides, a short extension of EU membership may be a price worth paying.
"This is not about unpicking Brexit, but about delivering the best Brexit possible."