The Institute for Government said it did not believe that Boris Johnson's Brexit plans surrounding the Irish border would be deliverable by December 2020.
It warned that if Johnson does not ensure the measures are in place by the end of the transition period - expected to be in 11 months time according to the prime minister - it could face a court case at the European Court of Justice.
The think-tank warns that the deal does not have the support of any Northern Irish parties and looks "impossible" to implement the proposed changes in time to reach Johnson's deadline.
It therefore warns that the failure to comply with the agreement would see the European Commission taking the government to court.
"The government will almost certainly be unable to implement the Northern Ireland Protocol by December 2020.
"This could result in the UK being taken to the ECJ and cause practical and political disruption in Northern Ireland.
"The protocol involves complex customs arrangements and regulatory and customs checks between Northern Ireland and Great Britain - but key details are yet to be decided by the Joint Committee established in the Withdrawal Agreement and not yet up and running.
"Failure to comply with the Withdrawal Agreement could see the European Commission begin infringement proceedings and the UK ending up at the ECJ."
The report also warns that Brexit will be far from completed when the process is ratified ahead of 31st January.
It says: "The UK will formally leave the EU at the end of January, and in that sense Brexit will be done, but many of the biggest Brexit jobs will be far from over.
"It will continue to dominate government for years to come. The prime minister may hope to end Brexit's dominance in the public debate after 31 January, but in Whitehall it will continue to be the biggest and most challenging task faced by a government in decades."
