Speaking on Wednesday in a plenary debate on the European Council's Brexit negotiating guidelines Barnier said the talks were "not about punishment but organising an orderly withdrawal."
He told MEPs, "There must be a foundation of trust for the talks. We will also need this for our future relationship with the UK. Our aim is to achieve an agreement. It is not about a good or bad deal, but an agreement with the EU, not against the EU."
He said the talks must start as soon as possible after the UK general election on 8 June, adding, "The fact that elections take place just before the talks start will give stability to the UK which is important."
Barnier also insisted, "We on the EU side will be transparent with the European Parliament throughout the entire process. I will be available to you to have exchanges with you before every negotiating round and same goes for my taskforce. We will also ensure full and great transparency on all the documents that will be published."
On Northern Ireland, Barnier told the plenary, "This is a unique situation so we need a unique solution. We will do all we can to find one that respects the Good Friday Agreement."
He told MEPs, "It must be understood that it is the UK who decided to leave, not the other way round. This decision has been taken but anyone who says that it can now be business as usual is just not telling the truth.
"The fact is that we now face the challenge of trying to unravel 44 years of our relations, including social, economic, legal, cultural and in many other areas, with the UK.
"The decision to leave was taken by the UK and we should not under-estimate the complexities of what this is going to involve.
"That is why we need to tell our citizens the truth and explain what Brexit really means, and also what it means if no deal is reached."
Barnier also warned, "The clock is ticking and there is not much time to conclude an agreement. There are consequences to Brexit and, also, lessons should be learned from this.
"That means we must listen to the reasons given by UK citizens for voting as they did - the rage, feelings of being left behind and excluded.
"We must, though, not confuse populism with popular sentiment and that is why we must draw lesson from Brexit."
He concluded, "However, Brexit is not only thing on the EU radar. There are many other challenges we also face, including border security, defence and the economy."