This is John Stuart Mill in his Principles of Political Economy, Book V, Chapter XI, Para 7 [Ashley edition p 950]:

Laissez-faire, in short, should be the general practice: every departure from it, unless required by some great good, is a certain evil.

This is William D. Grampp in his Economic Liberalism: , Volume II: The Classical View, Chapter 3, titled “Liberalism in the Great Century” [New York: Random House 1965]. The chapter begins:

The nineteenth century usually is thought to have been the greatest age of economic liberalism, greater than any other, from its origins in Stoicism down to the present time. Economists think of the century as the long afternoon of the ideology. They believe it then meant laissez-faire and that laissez-faire was the policy of Great Britain, the major economy of the world. In fact, the century was not like that, and historians have tried to tell us so. They have reported the many ways in which the British government intervened in the market. The historians also have said the intervention was inconsistent with liberalism. In this, they are in my opinion, mistaken. (Grampp 1965: 73)

From the introduction to the chapter he continues with his first section on “The Importance of the Nineteenth Century”. He lists six reasons, this is the fourth:

“(4) The first effort to make a comprehensive statement on its principles was in 1848.” (Grampp 1965: 74)

1848 was the date of publication of the first edition of Mill’s Principles. He makes what he means absolutely clear on the following page:

“John Stuart Mill was the first to do this – to enunciate a theory of policy – and he did it in his Principles, which was published in 1848.” (Grampp 1965: 75)

He makes clear its significance in the very first para in which he discusses the notion of laissez-faire.

“1. The idea of economic liberalism, which had been gathering force for centuries, came to their full power in the nineteenth century…. Liberalism had many different meanings to these people. But one generalization can be made. To almost all who believed in it and to some who did not, liberalism did not mean laissez faire – that is, it did not mean a policy of non-intervention by the government and of allowing the major economic decisions to be made on unregulated markets. The rejection of laissez faire was one of the few ideas on which there was nearly complete agreement among the economists and between them and the political leaders.” (Grampp 1965: 74)

In that quote from Mill, the most important word is “unless”, “unless required by some great good”. His Book V is “On the Role of Government”, and in the 185 pages that follow he makes clear just how extensive he believed that role is.