Mr Lamberts also claimed the withdrawal period, during which the UK will remain closely tied to the EU, could last beyond 2022 meaning another British General Election would take place before full withdrawal.
The Belgium politician made the powerful claims during an appearance on BBC Newsnight.
He said: “I think the first step towards the possible reentry of the United Kingdom to the European Union would be a withdrawal agreement because a withdrawal agreement creates a transition period practically nothing changes, the UK behaves as part of the European Union.
“During that transition period there’s a possibility for public opinion to shift.”
The Government has agreed the UK will remain in a transitional arrangement with the EU between March 29 2019, when Brexit officially takes place, and December 31 2020.
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During this period the UK will remain closely tied to Brussels, continuing to abide by EU trade deals and giving European nationals the same rights they had before Brexit.
Theresa May has already suggested this period could be extended beyond the end of 2020 to ensure a smooth departure.
During the BBC interview Mr Lamberts suggests the UK could still be in the transition phase in 2022, the latest possible date for the next general election.
He commented: “Is there a risk that the UK will not be fully out of the European Union by 2022, that is by the next General Election, yes.
“That risk, or for some opportunity, exists.”
Mr Lambert, a Belgium politician, sits on the European Parliament’s Brexit steering group which coordinates the Parliament’s response to the UK’s departure from the EU.
However, Mr Lambert warned the ‘Remain’ campaign could lose a second referendum, especially if it is held imminently.
He stated: “My message to those in the UK who wish to remain in the European Union is gambling on a second referendum right here and now may be the most risky and unfeasible path to get the desired result.
“I am not sure, at the moment, that public opinion has shifted to a point where a second referendum might generate a different result than the first.”
Mr Lambert also argued the UK has a legal duty to prevent the emergence of a hard-border between Ireland and Northern Ireland after Brexit.
He argued: “It’s true that the Irish border issue makes things more complicated than they could have been but they are caused by an international agreement freely entered into by the United Kingdom, namely the Good Friday Agreement.
“If the UK wants to abide by its word it has consequences for the way the UK can exit the European Union.”