The SNP leader said her party would seek to form a "progressive alliance" with Labour and the Liberal Democrats after the snap general election.
But she conceded that, unlike 2015, when she proposed a similar deal there was no prospect of Jeremy Corbyn's party winning enough seats to keep the Tories out.
Ms Sturgeon's comments, however, will be a major boost to the Prime Minister whose party exploited Ed Miliband's failure to rule out a deal with the SNP two years ago.
Nicola Sturgeon opens the door to a Left-wing pact to stop Theresa May from returning to Number 10
Scottish Tory leader Ruth Davidson warned such an alliance represented a "grave danger" that risked "plunging the UK into more instability and uncertainty".
The First Minister speaking on a visit to her MPs outside Parliament, said: "If the Parliamentary arithmetic lends itself to the SNP being part of a progressive alliance to keep the Tories out of Government then the SNP will seek to be part of that as we said in 2015.
"Now we are not sure that there are many people who think Labour will be in a position on their own or with anyone else to form a Government.
Politicians react to Theresa May's snap general election Wed, April 19, 2017
UK Prime Minister Theresa May has announced her intention to hold an early general election on June 8
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Theresa May has called for a snap general election
The SNP wants to break up the UK; it has no interest in making it work better
"There will be many people across England who will see that as a matter for regret."
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Ms Sturgeon insisted she was not floating a formal coalition but was discussing the prospect of supporting Labour and the LibDems on a vote by vote basis.
Denying she had "opened the door to a coalition" she added: "I don't think you will find anybody in any part of the UK who thinks it is.
"I was simply stating the fact as I did in 2015 that I don't want to see a Tory Government so if there I was a Parliamentary arithmetic that lent itself to keeping the Tories out of Government I would want to see that happen."
In the 2015 general election, Ms Sturgeon suggested the parties join forces to "lock David Cameron out of Downing Street".
Former Labour leader Mr Miliband's ill-fated election campaign was dogged by the question over whether he would do a deal with the Nationalists in a hung parliament.
Despite repeated Tory attacks he only ruled it out days before the poll.
But Scottish Labour leader Kezia Dugdale and Mr Corbyn have since rejected any deal.
Live from 10 Downing Street: Theresa May calls snap general election Tue, April 18, 2017
Theresa May has made the announcement to call for an early general election to be held on June 8 2017
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Britain's Prime Minister Theresa May speaks to the media outside 10 Downing Street, in central London
Earlier this week Ms Dugdale hit back at Neal Lawson, chairman of Left-wing think-tank Compass, after he called for a "progressive alliance".
Writing for the New Statesman she said the "desperate attempt to appease nationalism will hinder, not help, the fight for a fairer UK."
She added: "The SNP wants to break up the UK; it has no interest in making it work better."
A Scottish Lib Dem spokesman today said the party would not do any deal.
But Ms Davidson said: "Nicola Sturgeon has today made it clear: at this election, we face the grave danger of a weak Labour-SNP-Lib Dem alliance, headed by Jeremy Corbyn, plunging the UK into more instability and uncertainty.
Ms Sturgeon said her party would seek to form a ‘progressive alliance’ with Labour and the Lib Dems
"We know Jeremy Corbyn is 'absolutely fine' with the SNP's demand for an immediate second referendum.
"There is therefore a real danger of Mr Corbyn and Ms Sturgeon cooking up a deal to suit both their purposes."
Meanwhile, Mrs May took aim at Left-wing rivals in the Commons, telling MPs: "What do we know that the leader of the Labour Party, the leader of the Liberal Democrats and the leader of the Scottish Nationalists have in common?
"Corbyn, Farron and Sturgeon, they want to unite together to divide our country – and we will not let them do it."