The environmental group obtained 248 pages of classified documents from the TTIP trade talks, aimed at clinching a far-reaching EU-U.S. free trade deal, the BBC wrote.
Secrecy surrounding the talks has fuelled fears that U.S. corporations may erode Europe's consumer protections.
But the EU's top trade official denied any agenda to lower EU standards.
"I am simply not in the business of lowering standards," said EU Trade Commissioner Cecilia Malmstroem in her blog, after the Greenpeace leak was published.
TTIP's supporters say a deal would create many new business opportunities.
TTIP stands for Transatlantic Trade and Investment Partnership. It would harmonise regulations across a huge range of business sectors, providing a boost to exporters on both sides of the Atlantic.
The 13th TTIP negotiating round took place last week and the European Commission says it hopes to achieve a deal later this year. That could avoid any political risk posed by the U.S. presidential election in November.
The EU's chief negotiator, Ignacio Garcia Bercero, said some of Greenpeace's points were "flatly wrong," and stressed that the leaked text "is not a reflection of the outcome of the negotiation."
Mr Bercero said "it is not correct to say the U.S. is pushing for lowering of the level of protection in the EU."
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Corporate muscle
"These leaked documents confirm what we have been saying for a long time: TTIP would put corporations at the centre of policy-making, to the detriment of environment and public health," said Greenpeace EU director Jorgo Riss.
"We have known that the EU position was bad, now we see the U.S. position is even worse."
Greenpeace says the texts reveal that the U.S. wants to replace the EU's "precautionary principle" for potentially harmful products with the less strict U.S. approach, which aims to manage risks rather than avoid them altogether.
The precautionary principle can force a manufacturer to prove the absence of danger from a product.
It applies, for example, to genetically modified organisms (GMOs), whose possible risks to the ecosystem and the food chain are hotly debated.
The U.S. permits cultivation of more than 170 GM plants, whereas only one type – a maize variety – is approved for commercial cultivation in the EU.
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Mr Bercero denied any intention to weaken the precautionary principle.
Greenpeace says the TTIP texts do not refer to the global commitment to cut CO2 emissions, as agreed at the Paris Summit on global warming. Yet the European Commission had pledged to make environmental sustainability part of any TTIP deal.
There is also widespread concern in the EU about the role of commercial arbitration courts, independent of national courts, where firms can sue governments.
It is one of the thorniest issues in the TTIP talks.
There are fears that big U.S. corporations could put excessive legal pressure on some EU states. The threat of being sued could have a "chilling" effect on legislators, forcing them to water down welfare protections, critics argue.
Long way to go
The Commission and many politicians argue that TTIP would bring major benefits for the U.S. and Europe.
Last week U.S. President Barack Obama and Germany's Chancellor Angela Merkel called jointly for a TTIP deal to be concluded this year. However, thousands of protesters turned out in Hanover in a rally against the deal.
A final TTIP text would require approval by all 28 EU governments and the European Parliament.
A study by the Centre for Economic Policy Research (CEPR) estimated the potential gains for the EU as up to EUR 119 billion (GBP 94 billion; $137 billion) a year and EUR 95 billion for the U.S.
The EU's chief negotiator said the 13th round of talks covered 97% of current tariff barriers, which cost EU exporters more than EUR 3.5 billion annually.
But Mr Bercero added that "there is still a lot of work to be done."
They were very far from agreement on public procurement, he explained. The EU wants U.S. authorities to give European companies much more market access to compete for public contracts.
Among other tough issues, he said, were: the beef market, the services sector and the EU's protected geographical labels, such as Champagne and Gorgonzola.
