Lamberts damning warning comes just ahead of a UK House of Commons vote on the Withdrawal Agreement, which many observers expect May to lose.
Such an outcome would, May has conceded, cast the UK into “unchartered territory.”
The Withdrawal Agreement was thrashed out between the EU and UK ahead of Britain’s planned exit from the bloc on 29 March.
Lamberts, who co-leads the Greens/EFA grouping in the European parliament, has joined those warning of the consequences of the failure to back the deal, saying, "A no-deal Brexit would make the UK a rogue state, if the British government reneges on its commitments under the Good Friday Agreement and its financial obligations, it would no longer be seen as a reliable partner on the world stage.”
"The best deal the UK could possibly get is the one it currently has as an EU member. If the UK Houses of Parliament cannot come to an agreement on Brexit, then the question must be given back to the public in the form of a people's vote,” argues the Flemish member.
On Monday, the EU issued a letter to the British government spelling out a series of reassurances on the Irish backstop.
The letter, issued jointly by Donald Tusk and Jean-Claude Juncker, the presidents of the European Council and Commission respectively, stresses that the backstop is not the EU’s preferred solution to avoiding a hard border, that it does not undermine the Good Friday Agreement, nor is it part of any covert attempt by the EU to "annex" Northern Ireland.
However, the letter insists that there can be no renegotiation of the backstop.
"A no-deal Brexit would make the UK a rogue state, if the British government reneges on its commitments under the Good Friday Agreement and its financial obligations, it would no longer be seen as a reliable partner on the world stage” Philippe Lamberts MEP
The EU letter does not contain anything substantially new on the backstop. Mostly it just amplifies the assurances about the backstop only being temporary that were set out by the EU 27 in conclusions their summit in December.
But Tusk and Juncker do stress that they are serious about trying to find technical solutions to the border issue that would make the backstop redundant.
It reads, “The European commission also shares your intentions for the future relationship to be in place as quickly as possible. Given our joint commitment to using best endeavours to conclude before the end of 2020 a subsequent agreement, which supersedes the protocol (that is, the backstop) in whole or in part, the commission is determined to give priority in our work programme to the discussion of proposals that might replace the backstop with alternative arrangements.
“In this context, facilitative arrangements and technologies will be considered. Any arrangements which supersede the protocol are not required to replicate its provisions in any respect, provided that the underlying objectives continue to be met.”
The EU letter goes on to say that Brexit is a “source of uncertainty and disruption.
“In these challenging times, we therefore share with you the determination to create as much certainty and clarity as possible for citizens and companies in a situation where a member state leaves the European Union after more than four decades of closest economic and political integration.
“We are not in a position to agree to anything that changes or is inconsistent with the Withdrawal Agreement.”
Juncker and Tusk state, “In response to your concern about the timetable, we would like to make it clear that both of us will be prepared to sign the Withdrawal Agreement as soon as the meaningful vote has passed in the UK Parliament.
“This will allow preparations for the future partnership with the UK immediately thereafter to ensure that negotiations can start as soon as possible after the withdrawal of the UK from the European Union.”
On Monday, Downing Street also released a letter from Theresa May to the EU chiefs as part of the choreographed exchange, requesting further assurances on the backstop.
May wrote: "The clarifications and undertakings proposed in this letter are consistent with the letter and spirit of the deal we have reached, but would be further reassurance that the fears that some hold on both sides are misplaced."
May has sought further reassurances and clarifications on the backstop in a frantic bid to convince Conservative MPs to back the treaty.
Keir Starmer, Shadow Brexit Secretary for the UK Labour party, responding to the letters, said, “The Prime Minister has once again failed to deliver.
“This is a long way from the significant and legally effective commitment the Prime Minister promised last month. It is a reiteration of the EU’s existing position. Once again, nothing has changed.”