The UK government has demonstrated a "dereliction of duty" by failing to plan properly for country's split from the EU, John McDonnell argued on Tuesday (15 November).
Labour's shadow chancellor launched the attack against Theresa May and her ministers after a leaked memo claimed the government had no single Brexit plan and that Whitehall was working on 500 projects related to the issue, while needing to hire 30,000 extra civil servants.
Number 10 has since distanced itself from the "unsolicited" memo, which apparently came from an accountancy firm and was published in The Times newspaper.
"What we saw from the memo today simply confirms what we all suspected because of the various leaks that have been coming from the government already," McDonnell told a London audience.
"The issue is it isn't just about staffing, the memo is confirming what we knew, which is ministers arguing among themselves all the time, no common view of the way forward and no shared vision of the future for our country."
He added: "If we need the staff, we need to employ them, whether it's 30,000 or below. I just find it staggering that you have a government facing a momentum decision...and they haven't even staffed up. It's dereliction of government duty."
But despite McDonnell's concerns over the government's Brexit planning, he stressed Labour would not block a vote on Article 50 – the official mechanism to split from the EU – if MPs are given a say on the issue.
"To do so would put us against the majority will of the British people and on the side of certain corporate elites, who have always had the British people at the back of the queue," he said.
"The question of what Brexit means will be fought in the months and years through the negotiations and in the next election when we will define what a post-Brexit Britain looks like. It is time we all were more positive about Brexit, Labour wants to see an ambitious Brexit Britain."
McDonnell's speech came just over a week before Chancellor Philip Hammond unveils his Autumn Statement to MPs. The shadow chancellor urged Hammond to leave "the gimmicks on the shelf alongside George Osborne".
"To take responsibility, Hammond now needs to get a grip," McDonnell said.
"Philip Hammond now must come clean to the British people. He, like his Cabinet colleagues, supported George Osborne's calamitous economic choices. Worse, as Shadow Chief Secretary to the Treasury, he helped draw them up. He cannot duck his responsibilities, but he can at least attempt to undo the damage.
"That means no more targets that cannot be met. He needs, instead, to set out how he will place the government's own finances on the stable foundation that was so lacking from his predecessor.
"Abandoning Osborne's fiscal framework was the right decision, since the framework had failed so abysmally.
"But in typical fashion, the Conservative leadership have tried to put the blame for their shift on Brexit. They'll blame anyone and anything instead of accepting their responsibilities."
UPDATE: 14:35 GMT
Conservative MP Dominic Raab has responded to McDonnell's speech. "Labour have absolutely zero credibility when it comes to safeguarding our economy," he said.
"But what is truly astonishing is that John McDonnell completely failed to mention immigration, or control of our borders, in his vision of post-Brexit Britain.
"This is yet more evidence of how out of touch Labour are with the values of working people in our country.
"Only the Conservative Party can deliver the best possible deal as we leave the EU - and that must mean controls on the number of people that come to Britain."