Barack Obama overruled the advice of administration lawyers in deciding the US could continue participating in the Libya conflict without congressional approval, The New York Times reports.
The White House insists the president did not need congressional approval to authorise US support for Nato's mission, because the military campaign is limited in scope.
Critics argue the action violates a Vietnam War-era law limiting military action without congressional approval to 60 days.
The newspaper report said Pentagon General Counsel Jeh Johnson and acting head of the justice department's Office of Legal Counsel Caroline Krass had advised Mr Obama that the US involvement in the Libya air campaign constituted "hostilities".
But the US president opted to follow the advice of White House counsel Robert Bauer and state department legal adviser Harold Koh, who argued the US involvement fell short of "hostilities", the paper said.
US presidents can override the legal conclusions of the Office of Legal Counsel, but it is very rare for that to happen, analysts say.
The War Powers Resolution of 1973 states Congress must authorise participation in hostilities longer than 60 days, although the president can seek a 30-day extension.
Members of Congress have accused Mr Obama of violating that law since 20 May, when the 60-day deadline ended. Sunday marks 90 days since the US joined the Nato-led no-fly zone mission over Libya.
In a 32-page document delivered to Congress this week, the White House said that US forces involved in the Nato campaign were merely playing a supporting role.
That role, it said, did not match the definition of "hostilities" as described under the War Powers Resolution of 1973.
"US military operations are distinct from the kind of 'hostilities' contemplated by the resolution's 60-day termination provision," it said.
White House spokesman Eric Schultz said the law in question had been the subject of fierce debate.
There was a "robust process through which the president received the advice he relied on in determining the application" of the War Powers Resolution, said Mr Schultz on Thursday.
"It should come as no surprise that there would be some disagreements, even within an administration, regarding the application of a statute that is nearly 40 years old to a unique and evolving conflict. Those disagreements are ordinary and healthy," he added.
The revelation that key administration officials had wrangled over the legal implications of the Libya crisis could intensify anger in Congress over continued US participation in the conflict that is said to be costing the US some $10m a day, correspondents say.
On Thursday, John Boehner, the Republican speaker of the House of Representatives said: "The White House says there are no hostilities taking place. Yet we've got drone attacks under way.
"We're spending $10 million a day. We're part of an effort to drop bombs on Gaddafi's compounds. It just doesn't pass the straight-face test, in my view, that we're not in the midst of hostilities."
Libya is expected to be among the issues Mr Boehner and Mr Obama will discuss this weekend as they play a round of golf at an undisclosed location.
The US role in Libya involves helping Nato aircraft with refuelling operations and assisting with intelligence-gathering, said the White House.
The Obama administration insists that the US is not engaged in sustained fighting or "active exchanges of fire with hostile forces" that put US troops at risk.
Under the US constitution, the power to declare war lies with Congress.
A bipartisan group of US lawmakers is suing Mr Obama in federal court for taking military action in Libya without authorisation from Congress.
The lawsuit alleges the president has violated the US constitution by bypassing Congress.