Speaking in Strasbourg on Tuesday, Guy Verhofstadt also said he "welcomed" the announcement by UK Prime Minister Theresa May that article 50, the process that will pave the way for Britain to leave the EU, will be triggered by next March.
At the Conservative party conference on Sunday, May also said Britain would not emulate Norway or Switzerland, but forge a relationship with the EU as a fully independent, sovereign country.
The Prime Minister also promised a bill to scrap the 1972 act that took the UK into the Union and to transfer existing EU law into British law.
ALDE group leader Verhofstadt said May's announcement gave "clarity" to Britain's EU exit, but added, "There can be no pre-negotiations until Article 50 is triggered."
"I hope the negotiations can be concluded before the next European elections. That is important for everyone, including both EU and UK citizens. I cannot imagine a situation where the talks are still not concluded and the UK is electing members for the next elections in 2019."
The Belgian MEP added, "We will prepare for the negotiations. However, it is not up to us to tell the UK what to do."
He also repeated his position regarding the UK's access to the single market, saying, "I hope the relationship can remain close. That is important for the 48 per cent of Britons who voted to remain the EU. But, whatever kind of relationship it is in the future, we cannot impinge on the four fundamental freedoms of the EU. If you start trying to split them that will destroy the EU."
His comments were largely echoed by EPP group leader Manfred Weber, who warned that the UK "faces a great problem" in the upcoming Brexit talks.
The German deputy welcomed May's announcement on article 50, which, he said, offered "clear timing and is very positive."
He added, "This was overdue and we in the EU have had to wait too long to hear how the EU will react to its referendum. However, her announcement has left open how she sees the future partnership with the EU so we need further clarity on that.
"What must be clear is that the four fundamental freedoms of the EU are not up for debate. We will try to offer the Brits a good deal but these are part of the EU's DNA and we will defend them and I cannot see where we can compromise on that."
Meanwhile, S&D group Chair Gianni Pittella voiced concern about May's pledge to repeal EU legislation, warning, "This will have very serious consequences for all EU citizens living and working in the UK."