Michel Barnier has written to Mark Francois and told the Chair of the Eurosceptic European Research Group of Tory MPs that Brexit isn't worth it.
Mr Francois had sent the EU's chief negotiator an open letter, which he titled a "missive from a free country", on June 29.
"While nobody has been able to demonstrate to me the added value of leaving the most integrated economic and free trade area in the world, I have always respected the UK's decision to withdraw from the EU," Mr Barnier said in reply.
"In this same spirit we approach the ongoing negotiations with your great - and indeed free - country."
Mr Barnier warned Mr Francois that the EU would hold Boris Johnson to the joint Political Declaration and that Brussels was adamant that the European Court of Justice must have the final say on the interpretation of EU law.
The Political Declaration, which is a non-binding document setting out the broad outline of the free trade agreement, was "agreed by your prime minister and voted for by the House of Commons,including yourself," Mr Barnier added.
He said the level playing field guarantees the EU demands were necessary. Mr Barnier said that the EU would not agree a free trade deal without an agreement on fishing and described progress in negotiations as "disappointing".
"We will continue to work with determination to conclude the negotiations with success as we continue to believe that this can be done despite the short time available, which is the choice of your government." Mr Barnier said, referring to the UK's refusal to extend the transition period.
The French politician and commission official told Mr Francois it was a "pleasure" to hear from UK elected representatives.
"Having been an elected representative for many years myself I value the crucial role you play in scrutinising and holding your government to account," he said
In response, Mr Francois told the Telegraph that he was grateful for the "charming billet doux". He said, "As he rightly acknowledged, we are now a free country - and indeed very happy to be so."
He added that the political declaration was "indicative and not legally binding - as Michel knows well. Nevertheless, I’m delighted that further talks are now agreed. As he often reminds us, 'the clock is ticking.'”
UK and EU officials met in London this week and will meet again in Brussels next week as the try to break the deadlock in the trade negotiations.
"This week’s discussions confirm that significant divergences remain between the EU and GB. We will continue working with patience, respect and determination," Mr Barnier tweeted on Thursday.