The Harry Potter author, who lives in Edinburgh, entered the referendum debate yesterday with an article on her website highlighting her concerns about the economy and the future of medical research in an independent Scotland.
Her donation is the biggest yet for the pro-Union campaign.
Yes Scotland has benefited from £2.5 million from Colin and Chris Weir, who won £161m in the EuroMillions lottery in 2011.
Writing in her blog, Rowling said that, while she is “no fan of the current Westminster government”, she has concerns about the economic risks of independence.
“My hesitance at embracing independence has nothing to do with lack of belief in Scotland’s remarkable people or its achievements,” she wrote.
“The simple truth is that Scotland is subject to the same 21st century pressures as the rest of the world. It must compete in the same global markets, defend itself from the same threats and navigate what still feels like a fragile economic recovery.
“The more I listen to the Yes campaign, the more I worry about its minimisation and even denial of risks.”
The author, who was born in the west country in England but has lived in Scotland for 21 years, said there is a “fringe of nationalists who like to demonise anyone who is not blindly and unquestionably pro-independence”.
“I suspect, notwithstanding the fact that I’ve lived in Scotland for 21 years and plan to remain here for the rest of my life, that they might judge me ‘insufficiently Scottish’ to have a valid view,” she said.
Referring to characters from her Harry Potter books, she added: “When people try to make this debate about the purity of your lineage, things start getting a little Death Eaterish for my taste.”
A Death Eater is a follower of the evil Lord Voldemort in the Harry Potter series.
In an emotive comment, she added: “By residence, marriage, and out of gratitude for what this country has given me, my allegiance is wholly to Scotland and it is in that spirit that I have been listening to the months of arguments and counter-arguments.”
Criticism of the author quickly flooded in on Twitter, with foul-mouthed tweets condemning the author’s political position.
Last night, the Office of the Scottish Charity Regulator said it was making “urgent” inquiries into one tweet, which appeared to have been sent by the Edinburgh-based charity the Dignity Project.
The tweet said: “What a #bitch after we gave her shelter in our city when she was a single mum.”
The charity’s website published a disclaimer saying it was not responsible for the tweet claiming its account had been hacked.
Shadow Scottish secretary Margaret Curran welcomed Ms Rowling’s stance on independence and criticised the “vile abuse”.
Ms Curran said: “I think it’s really one of the most disturbing elements of this campaign that people are subjected to such abuse online. Particularly when we see voices from beyond the political world speaking and they are subjected to vile abuse.
“This has to stop. It’s time to let people speak their minds freely without any fear of retribution.”
Former Scotland secretary Baroness Liddell also hit out at the abuse, saying: “This is a shame on my country and I am appalled and I urge the First Minister to stand up for the decent people of Scotland who will have nothing to do with this.”
The Prime Minister’s spokesman also welcomed the support from the author and criticised online comments.
Speaking at a regular Westminster briefing, David Cameron’s official spokesman said: “There is never any place for abusive behaviour in whatever sphere of life.”
A spokesman for Yes Scotland said: “While we may disagree with her views, we of course completely respect JK Rowling and her right to express her opinion on the referendum and donate to the No campaign.”
The author signalled her support for Better Together publicly earlier this year when she attended the campaign’s fundraising night with comedian Eddie Izzard in Edinburgh.
Ms Rowling previously donated £10m to set up a clinic at Edinburgh University to research treatments for multiple sclerosis, the degenerative disease that killed her mother.
“Having put a large amount of money into multiple sclerosis research here, I was worried to see an open letter from all five of Scotland’s medical schools expressing ‘grave concerns’ that independence could jeopardise what is currently Scotland’s world-class performance in this area,” she wrote.
She concluded: “If the majority of people in Scotland want independence, I truly hope that it is a resounding success.
“Whatever the outcome of the referendum on 18 September, it will be a historic moment for Scotland. I just hope with all my heart that we never have cause to look back and feel that we made a historically bad mistake.”
Scottish Conservative chief whip John Lamont said about the tweets: “Some of the comments made about her are abhorrent and if they were made public in the street would surely be a criminal offence.”
When the stars come out
Supporters of a “YES” vote include:
Sir Sean Connery
Hardeep Singh Kohli
Elaine C Smith
BETTER TOGETHER supporters include:
Sir Alex Ferguson