Rachel Reeves has accused the Chancellor of having "fanned the flames" of the falling value of the pound by dropping hints at further "unfunded" tax cuts.
The Shadow Chancellor said the Tories had fuelled concern in spooked markets in the wake of Friday's explosive mini-Budget after Kwasi Kwarteng signalled more taxes could be slashed on Sunday.
The pound plunged to an all-time low against the US dollar this morning amid hammered market confidence over the Government's economic plans.
Sterling hit its lowest level against the dollar since 1971, falling by more than 4% to just 1.03 dollars in early Asia trading before it regained some ground to about 1.07 dollars early on Monday.
Former Chancellor George Osborne branded the Government's plans to slash taxes and increase borrowing as "schizophrenic" amid growing signs of Tory nerves.
Ms Reeves told BBC Radio 4's Today programme: "It is incredibly concerning.
"I think many people had hoped over the weekend things would calm down but I do think the Chancellor sort of fanned the flames on Sunday in suggesting there may be more stimulus, more unfunded tax cuts, which has resulted overnight in the pound falling to an all-time low against the dollar."
In a series of interviews, Ms Reeves accused PM Liz Truss and Mr Kwarteng of "behaving like two gamblers in a casino chasing a losing run".
She added: "They're not gambling with their own money, they're gambling with all our money, and it's reckless and it's irresponsible as well as being grossly unfair."
But on Monday No 10 declined to comment and remained committed to the economic strategy set out by Mr Kwarteng on Friday.
The Prime Minister's official spokesman said on Monday: "I think that the Chancellor has made clear that he doesn't comment on the movements around the market and that goes the same for the prime minister.
"The UK with the second lowest debt-to-GDP ratio in the G7 is investing in its future. That's through a growth plan while remaining fiscally responsible and committed to driving down debt in the medium term.
"The growth plan, as you know, includes fundamental supply side reforms to deliver higher and sustainable growth for the long term, and that is our focus."
Downing Street also declined to say whether the Chancellor would be meeting with the Governor of the Bank of England as a matter of urgency to discuss the sudden drop in the pound.
"I know he speaks regularly to the Governor of the Bank of England. I don't know when the next conversation is scheduled to be," they added.
Mr Osborne, who ran the Treasury between 2010 and 2016, told Channel 4 that "you can't just borrow your way to a low-tax economy".
"Fundamentally, the schizophrenia has to be resolved - you can't have small-state taxes and big-state spending," he said.
Former Tory deputy PM Damian Green told GB News "there's more to conservatism than tax cutting", and said with a general election in two years things "have to happen quickly".
The Lib Dems called for the Government to urgently recall Parliament to deal with the fallout from the mini-Budget.
Lib Dem Chief Whip Wendy Chamberlain said: “Last week the Chancellor announced a shambolic budget that gave huge unfunded tax cuts to big banks and the wealthiest while leaving struggling families and pensioners in the cold.
"As a result we are seeing the pound plummet into free fall as the markets give the Conservatives a damning vote of no confidence.
“The Government must urgently recall Parliament so Kwasi Kwarteng can fix this failed budget, before it does any more damage to our economy and people’s livelihoods."
Shadow Business Secretary Jonathan Reynolds said the Chancellor's mini Budget will be "remembered as a moment of economic disaster".
He added: "Fundamentally what we have seen from the events on Friday... the political effect of Friday without being overdramatic is not dissimilar in principle to say Black Wednesday."
Pressed on calls by the Liberal Democrats on whether Parliament should be recalled in the wake of the "disastrous" mini-Budget, Mr Reynolds said it was only in the government's power to do so.
"I'll see what Liberal Democrats have said about the government and the decisions they've made in Parliament," he said.
