Mr Francois, chairman of the European Research Group of eurosceptic Tory MPs, spoke out after a punchy exchange of letters with Mr Barnier.
The EU’s chief Brexit negotiator said he respected the UK’s move to quit the bloc but couldn’t see the logic in the decision.
The Frenchman wrote: “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.
“In this spirit, the EU negotiated the Withdrawal Agreement with your government. In this same spirit, we approach the ongoing negotiations with your great – and indeed free – country.”