David Cameron's failure to plan for Brexit amounted to "gross negligence" and Theresa May must now significantly boost funding for the Foreign Office, an influential committee of MPs has said.
The previous government's lack of contingency planning has exacerbated uncertainty and made the task facing the new Prime Minister's administration "substantially more difficult", the Foreign Affairs Committee report said.
The cross-party committee said it was essential that Foreign Secretary Boris Johnson, Brexit Secretary David Davis and International Trade Secretary Liam Fox formed an "effective triumvirate" as soon as possible, with clear division of responsibilities.
The report was highly critical about the policy adopted under Mr Cameron's premiership of not fully preparing for the possibility of a victory for the Brexit campaign in the European Union referendum.
"Since the referendum, the extent of the Government's lack of preparation for a potential Leave vote has become more evident," the report said.
"In the light of the appointment of the new Prime Minister on 13 July, the previous government's confidence that basic planning for the practicalities of implementing Brexit could be undertaken at a leisurely pace after the vote now appears at best naïve and at worst negligent.
"The previous government's considered view not to instruct key departments including the FCO (Foreign and Commonwealth Office) to plan for the possibility that the electorate would vote to leave the EU amounted to gross negligence.
"It has exacerbated post-referendum uncertainty both within the UK and amongst key international partners, and made the task now facing the new government substantially more difficult.
"The lack of contingency planning inevitably means that the Government's plans are tentative and just emerging."
The committee urged Mrs May to commit to a sizeable boost to the funding of the FCO "commensurate with the enormity of the task it now faces" and warned against stripping senior officials away from the "already over-stretched" department to focus on Brexit without replacing them.
Committee chairman Crispin Blunt said: "We want to see the FCO working effectively with the new Department for International Trade and the Department for Exiting the European Union.
"Our security, prosperity, values and democracy will depend on the strength of these key departments and their working relationships.
"The Brexit challenge requires a fully staffed and resourced FCO.
"A commitment to scaling up resources and personnel would give a clear signal to allies of our priorities."
At her first Prime Minister's Questions, Mrs May was challenged by Liberal Democrat leader Tim Farron over reports that the new Brexit unit would be hiring lawyers at a cost of up to £5,000 a day each.
Mrs May said it was "absolutely right" to create the new Brexit department and it would need "the expertise that will enable it to undertake the negotiations".