Mrs May further shattered the glass ceiling when she became only the second female Prime Minister but now she has fallen to the concrete floor below.
According to the survey of 1,119 members, the PM now has a net satisfaction of -73.5 - down from -51.2 from the end of March.
That figure means she is now even more unpopular than Transport Secretary Chris Grayling, who sits just above her on 71.1.
The PM has taken a battering over her failure to deliver Brexit and her refusal to name a departure date.
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On the other end of the scale, her one-time rival for the top job, Andrea Leadsom, has the highest net satisfaction among members, at 40.6.
The Brexit-supporting Leader of the House has been tipped as a future leader - long with Liz Truss, who came second in the popularity stakes with a net satisfaction of 38.0.
The results of the most popular member of the Cabinet found Health Secretary Jeremy Hunt complete the top three - which will do no harm to his own leadership ambitions.
Mrs May is still trying to get her Brexit deal through Parliament but senior Tories are reportedly plotting to change party rules so that they can oust her before she can resign.
Officers of the Tory backbench 1922 Committee met in Westminster on Tuesday amid growing pressure for the PM to name the date of her departure.
But Health Secretary Matt Hancock warned that changing the PM would not resolve the deadlock in Parliament over Brexit.
Speaking at a TaxPayers' Alliance launch event in London, he said he still hoped it would be possible to get a majority for a deal.
"Changing the prime minister will not change what we need to do to deliver Brexit," he said.
"I hope the House of Commons will come to a majority to be able to deliver the result on the referendum.”
The Tories are predicted to suffer heavy losses in both the local elections and European elections in May.