In his last pre-Budget speech before stepping down as shadow chancellor, Mr McDonnell also urged the prime minister and the chancellor, Rishi Sunak, to “get a grip” of the situation and reassure both the public and financial markets.
It comes as the numbers tested positive of the virus in the UK jumped to more than 300 as the prime minister held a Cobra meeting to reassess the government’s response.
Speaking in central London, Mr McDonnell said the “immediate and pressing” challenge of the Budget on Wednesday must be to ensure the NHS has the necessary resources to defeat coronavirus.
But, he added: “The natural focus on the coronavirus should not be a reason or an excuse for not addressing the equally serious and dangerous threats from the social emergency created by a decade of decline and the climate change crisis we also face.
“The scale of government intervention on both these emergencies mooted so far fails to recognise the significance of the threats we face from the rundown of our public services and from the running out of time to halt the climate crisis.”
After his speech, he went on: “What I’m worried about in this Budget is that the virus is used to put key decisions off and that we just have another round of consultations. To be honest, I think there’s an element an consultation fatigue – we need some decisions at the moment.”
Addressing No 10’s response to the virus, Mr McDonnell said now was “not the time for any party politics or partisan behaviour”, but suggested Mr Johnson should have convened the government’s emergency committee Cobra “sooner”.
The senior ally of Jeremy Corbyn also claimed the ministers had been too slow to respond in the face of threat to economic confidence, and the “delaying of any statement until the Budget date itself doesn’t seem to appreciate the urgency of action” that is needed.
“What concerned me, and still does, is the tardiness of the chancellor in seeking to reassure the public and markets more comprehensively that the government stands ready,” he said.
He continued: “Whatever criticisms people may have of Gordon Brown’s policy strategy in the banking crisis, nobody can question the international leadership he showed and the focus and determination he brought to dealing with those events globally.
“I regret that we have not seen that leadership, commitment, indeed political, diplomatic and indeed managerial ability from either the prime minister or the chancellor. I just say gently, someone needs to get a grip.”
The shadow chancellor also suggested that while the UK’s negotiations with the EU over a future trading agreement were important, they should be treated as a “secondary” issue while attempts are made to contain the virus.
Asked by The Independent whether the Brexit talks should be suspended while the government deals with the outbreak of covid-19, he replied: “We are trying to work in a non-partisan, cross-party basis.
“But we are hearing increasingly noises from businesses and others about their problems they will confront in preparedness for Brexit implementation given the fact they’ve also got to address the threat of this virus outbreak.
“In addition to that, others have questioned the limitations of the bandwidth within government to be able deal with a global crisis and the negotiations themselves on the timetable the government has now set.
“That’s a matter now we’ll need to work on a cross-party basis to achieve some form of agreement – what we have to do at the moment is saving lives. Brexit, although significant and important, has to be a secondary matter to keep our people safe.”
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