Opposition parties and the small Northern Irish party that props up Ms May's minority Government are furious that it only provided an outline of the legal basis for its Brexit deal after Parliament voted to be given the full advice.
Her spokesman said the cabinet had discussed the motion on Tuesday but maintained that ministers must be able to obtain candid legal advice "without fear that it will be immediately published".
Shadow chancellor John McDonnell said: "The contempt the Tories have shown Parliament and the people of this country demonstrates that they are not fit for the office they hold".
It is the first time in modern history that any Government has been found in contempt and means the highly sensitive advice provided by Attorney General Geoffrey Cox will be published, in contravention of long-standing practice.
British Prime Minister Theresa May's government fought on Monday to defend its Brexit deal by outlining the legal basis for Parliament to support its plan to leave the European Union, but instead seemed to fan the flames of rebellion.
Labour demanded that it be done before next Tuesday when the vote on Mrs May's Brexit deal takes place.
"The legal summary document is worse than we feared: The backstop customs union is indefinite, the U.K. would be a rule taker and the European Court [of Justice] is in charge of our destiny, rather than the sovereign U.K. Parliament", former Brexit minister David Davis said.
"The numbers in the Houses of Parliament look pretty formidable for Theresa May", said Alan Wager, a research associate at the U.K.at the Changing Europe think tank.
Labour Party Brexit spokesman Keir Starmer said the contempt finding was "unprecedented", and the government said it would now publish the advice.
Advocate general Manuel Campos Sanchez-Bordona's non-binding opinion said Article 50 allows the "unilateral revocation of the notification of the intention to withdraw from the European Union, until such time as the Withdrawal Agreement is formally concluded".
The guidance is not binding on the Luxembourg court, which is considering the issue in response to a request from British parliamentarians.
He told the Commons: "That is contempt".
He told the Commons Treasury Committee increased tariff prices, import costs and a collapse in the value of the pound would send food prices soaring "quite quickly". Rejecting it would leave the United Kingdom facing the prospect of a chaotic "no-deal" Brexit, but May's chances of winning majority backing for the deal appear slim.
"I never said this deal was ideal, it was never going to be".
For starters, there has been little agreement in parliament on much of anything except that no one likes the deal May secured.
Commons leader Andrea Leadsom confirmed the u-turn over the legal advice after MPs decided her ministers were in "contempt" of Parliament.
"What we break now may be very hard to fix later", Leadsom said.
And she will insist: "This is the deal that delivers for the British people".
Opening debate on the deal she struck in Brussels last month, Mrs May warned: "Don't imagine that if we vote this down another deal is going to miraculously appear".