The Shadow Chancellor insisted she’s working on a plan to pay for two million freed-up NHS appointments every year and 700,000 new emergency dental spots.
Senior ministers warned that to cover the costs Labour would hike taxes on hardworking Brits.
Ms Reeves had previously pledged billions for the NHS and school breakfast clubs paid for by abolishing the non-dom tax status.
But Chancellor Jeremy Hunt got one-up on his opposite after he tore up the tax status at last week’s Spring Budget to help pay for a 2p cut to National Insurance.
The move left a humiliated Labour shy of billions for its flagship promise to voters.
Speaking on yesterday’s Laura Kuenssberg show on the BBC, Ms Reeves vowed to “make sure our sums add up”.
She said: “It is utterly humiliating that it’s taken the Conservatives this long to abolish a gaping loophole in our tax system.
“The Government made this announcement on Wednesday and I’m now going through all of the Government documents to identify the funding stream to honour those commitments.
“Your viewers would want to know that I’m doing the work properly and I’m not just plucking numbers out of the air.”
Senior ministers blasted the Shadow Chancellor for making now unfunded spending promises.
Chief Secretary to the Treasury Laura Trott said: “For five days Labour have been unable to say how they’re going to pay for their unfunded spending commitments.
“That’s because they don’t have a plan to pay for it and that means higher taxes, taking us back to square one.”
Ms Trott challenged Ms Reeves to immediately “tell Britain why Labour believe hardworking families should be taxed twice for work”.
She added: “Only Rishi Sunak and the Conservatives have a long-term plan to end the double tax on work.
“Labour has no plan and will take us back to square one with higher taxes.”
On Sunday Ms Reeves also hinted that a Labour government would put Whitehall spending splurges above slashing taxes.
Asked whether she would use a surplus in Treasury coffers to take a penny of income tax or spend it on colleges and courts, the Shadow Chancellor responded: “The thing that got me into politics in the first place was education and schools.
“For me, giving all children a good start in life is something that I feel absolutely passionate about.”