The novel, set in a small town in the West Country, shifted 124,603 copies in its first three days, outselling closet rival, 1356 by Bernard Cornwell, by ten times.
The book also sold 20 times more copies than the next new entry, Jackie Collins's The Power Trip, according to data from Nielsen BookScan.
The Casual Vacancy tells the story of a local election revealing stark divisions between people in the supposedly idyllic town of Pagford.
Despite a mixed response from the critics, Rowling's new book has proved a big hit with consumers.
It has become the fastest-selling hardback novel since Dan Brown's The Lost Symbol, which sold more than 550,000 in its first week of sales in 2009. The book is also the 15th biggest seller of the year so far.
The sales figures, however, are a far cry from those of Rowling's seven Harry Potter books, which enjoyed massive success in Britain.
"By way of comparison, the latest phenomenon in the book trade, Fifty Shades of Grey, sold 664,000 in its highest week of sales so far," Nielsen BookScan's David Walter said.
"JK Rowling's UK sales to date are of a value in excess of £238m, making her by far the highest-selling author in the UK since our records began [in 1998]."
The Casual Vacancy's release last Thursday prompted some rival books to be pushed back to avoid the title, including Ian Rankin's latest work being postponed until November.