King George V’s Royal Warrant declared that Princesses Helena Victoria and Marie Louise would drop the territorial designation “of Schleswig-Holstein” but would remain Their Highnesses Princess Helena Victoria and Princess Marie Louise.
See
Royal Styles and Titles of Great Britain: Documents
(scroll down to "Daughters of Princess Christian of Schleswig-Holstein (1917)")
This is an often overlooked case of British Princess titles in the female line of the British royal family.
King George V's general
proclamation of July 17, 1917 stated in part:
"We for Ourselves and for and on behalf of Our descendants and all other the descendants of Our said Grandmother Queen Victoria who are subjects of these Realms, relinquish and enjoin the discontinuance of the use of the Degrees, Styles, Dignities, Titles and Honours of Dukes and Duchesses of Saxony and Princes and Princesses of Saxe-Coburg and Gotha, and all other German Degrees, Styles, Dignities, Titles, Honours and Appellations to Us or to them heretofore belonging or appertaining."
In other words,
all descendants of Queen Victoria who were British subjects relinquished
all their German titles, names, and honors.
Helena Victoria and Marie Louise were grandchildren of Queen Victoria. They were born in the UK, and as the UK applied jus soli nationality until 1983, that meant they were British subjects at birth. Helena Victoria, who never married, therefore remained a British subject in 1917. (Marie Louise lost her British nationality automatically through marriage to a foreign man in 1891. I do not know whether she took steps to recover it after their divorce.)
So Helena Victoria, at least, relinquished her German title of Princess, according to George V's proclamation of 1917.
That means the title of Princess she used from 1917 onwards must have been a British one, granted by King George V's Royal Warrant. Announcement quoted from Gawin's link:
"The King has been pleased to direct that Royal Warrants shall be prepared declaring that Their Highnesses Princess Helena Victoria and Princess Marie Louise of Schleswig-Holstein shall henceforth be styled Their Highnesses Princess Helena Victoria and Princess Marie Louise respectively and whereas the Princesses of the Royal Family who bear the Title of Duchess of Saxony and who are his Subjects have at His Majesty's desire relinquished the said Title, He has been also pleased to direct that Royal Warrants shall be prepared declaring their present styles and making such corresponding alteration in their arms as may be necessary. "
The same would be true of Marie Louise if she regained her British nationality before 1917.