A prolific child performer, Fletcher's other West End credits include Jemima Potts in Chitty Chitty Bang Bang at the London Palladium and Jane Banks in Mary Poppins at the Prince Edward Theatre.
The actress, whose brother is Tom Fletcher from McFly, tweeted earlier today:
Also starting performances tonight are West End debutants Na-Young Jeon (Fantine), Rob Houchen (Marius) and Anton Zetterholm (Enjolras). They will be joined by Wendy Ferguson, who recently played Carlotta in The Phantom of the Opera, as Madame Thénardier.
Les Miserables originally opened in London at the Barbican Theatre on 8 October 1985, transferred to the Palace Theatre on 4 December 1985 and moved to its current home at the Queen's Theatre on 3 April 2004.
Adapted from Victor Hugo's epic novel of the same name, Les Miserables is written by Alain Boublil and Claude-Michel Schonberg with lyrics by Herbert Kretzmer and tells a story of love and bravery in 19th-century France.
The musical has been produced in 42 countries worldwide and translated into 21 languages.