Housing Secretary James Brokenshire vowed to make homelessness "a thing of the past" and the government has vowed to end rough sleeping by 2027.
But Labour says the plan is already "unravelling" as it's become clear no new money is being invested.
Homelessness has been on the rise for the past seven years, with around 4,750 people estimated to be sleeping rough on any given night in England in 2017.
Charities have welcomed the plan, but warned that it was "a step forward and not a total fix".
The Rough Sleeping Strategy will focus on preventing people from becoming homeless in the first place by offering a range of support.
Rough sleepers in England per 10,000 households
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Source: Ministry of Housing, Communities and Local Government
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But when questioned on BBC Radio 4's Today programme, Mr Brokenshire said half of the promised £100m had already been committed to homelessness and rough sleeping, with the other half "reprioritised" from existing budgets in his department.
"There are significant sums of money being focused and targeted," he said. "Half of that has already been committed to homelessness and rough sleeping.
"The other remaining half of this is money that's new to rough sleeping and homelessness, reflecting and recognising the priorities and importance of taxes."
He also denied that government policies were behind the rise in homelessness.
Shadow housing secretary John Healey said: "The government's rough sleeping plan has unravelled just hours after it was announced. It's now clear there is no additional money for the housing department to tackle the crisis of rough sleeping.
"Rough sleeping has more than doubled since 2010 thanks to decisions made by Tory ministers, but this feeble plan lacks any urgency."
He added that the next Labour government would end rough sleeping within its first term by making 8,000 homes available to those with a history of sleeping on the streets.
And Sarah Jones, shadow housing minister, said she was "shocked" it was not new money.