The parliamentary report said decision-making needs to be streamlined and other commitments cut back so attention can be focused on European Union withdrawal.
The Commons Public Accounts Committee (PAC) deputy chairman Sir Geoffrey Clifton-Brown, blasted the government for failing to reprioritise its work ahead of Brexit day in March 2019.
Pro-Brexit Tory MP Sir Geoffrey said: “Brexit is a byzantinely complicated task with the potential to become a damaging and unmanageable muddle.”
According to Sir Geoffrey, departments lack the “technical, project and senior leadership for Brexit alongside all other planned activity”.
In total 313 separate “workstreams”, which will need to be completed and focused on, ensuring plans to deal with them are “up to scratch”, have been identified by David Davis’s Department for Exiting the EU (DExDU), the report said.
But in December, Philip Rycroft, DExDU’s top civil servant told PAC that there was “a long road to go” to turn some of the plans into reality.
The report found DExDU has been “too slow,” and said: “[Plans] may not be sufficiently developed to enable implementation to start quickly.”
All departments have been urged to review their wider commitments by March, to determine which must be ditched or delayed, ahead of preparation for Brexit.
The report added: “It is clear that prioritisation has not been undertaken with the speed or on the scale needed and we have seen no evidence that departments have stopped any significant work.”
Neither the DExDU nor the Cabinet Office have a “credible” plan to recruit the skilled people needed to deliver Brexit, the report found.
Sir Geoffrey said: “It is one thing to identify the amount of work required to deliver Brexit. It is quite another to do it.
“It is concerning that government departments still have so far to go to put their plans into practice.”
The select committee has called on government officials to provide a formal update by June 1.