Brexit remains a "lose, lose" situation for both the UK and the European Union, according to the former European Commissioner for Cohesion and Reforms.
Michel Barnier made the comment a year on from the trade deal he helped to negotiate.
He led the European Union side in the talks which resulted in the Trade and Cooperation Agreement (TCA).
The Christmas Eve 2020 deal put in place the arrangements for continued trade with the UK outside the single market and customs union.
Mr Barnier said: "One year later, Brexit remains a lose-lose decision.
"Yet I remember a great negotiating team and our daily work for the unity of the 27.
"The European project deserves to be defended and reformed with the same unity and the same energy."
Following the signing of the agreement, Mr Barnier published his diary of the negotiations and launched an unsuccessful run for the French presidency.
One year later, Brexit remains a lose-lose decision.— Michel Barnier (@MichelBarnier) December 24, 2021
Yet I remember a great negotiating team and our daily work for the unity of the 27. The European project deserves to be defended and reformed with the same unity and the same energy. Happy Christmas and happy reading too ;-) pic.twitter.com/NxPKSKU9T0
Although the TCA meant the potential chaos of a no-deal Brexit was avoided, relations between the UK and EU have remained difficult.
Talks continue to try to resolve issues around Northern Ireland's trading arrangements and the UK has been locked in a bitter dispute with France over fishing rights.