Writing in The Times, the Prime Minister said it was important for the United Kingdom to “pull together”, stating that the SNP were trying to “force” her Government into agreeing to hold a vote before the country leaves the EU.
Mrs May said: “I do not agree with the SNP that now is the time to be talking about a second independence referendum.
“To do so now, while all our energies should be directed towards the negotiations with Europe, would make it more difficult to get the right deal for Scotland and the right deal for the UK as a whole.
“It is not something to which any responsible government could reasonably agree.”
She added: “The SNP is trying to force the UK Government to agree to something that is fundamentally unfair to the Scottish people. It wants to ask them to make a crucial decision without the necessary information.
“They would not know what the new partnership with the EU would look like, or what the alternative of an independent Scotland would be.
“It would simply not be fair.”
On Monday, Scottish First Minister Nicola Sturgeon announced plans to hold a second independence referendum, insisting a ballot on leaving the UK is necessary to allow Scotland to choose its own course and avoid a hard-line Tory Brexit.
However Theresa May rejected Ms Sturgeon’s calls on Thursday, saying “now is not the time”.