Because of declining health, his bitterness about Euryanthe's fate and a crushing workload at court, Weber entered a long compositional hiatus after the première of Euryanthe. Even the return to the beloved summer house at Hosterwitz for the spring and summer of 1824 failed to shake him out of this dormancy; he could generate no enthusiasm for Die drei Pintos (Mahler later completed it and gave the première in 1888), and the only piece he composed between autumn 1823 and January 1825 was a commissioned French romance (j292). The few professional pleasures the demoralized composer seemed to enjoy came from conducting, as he gave a memorable performance of Haydn's The Seasons at the old opera house in Dresden on 6 June 1824 and went at the end of the month to the Klopstock centenary celebration at Quedlinburg, where he conducted the ‘Eroica’ Symphony, his own Jubel-Ouvertüre, J.G. Naumann's Vater unser and the third part of Handel's Messiah. He then went to Marienbad on 8 July to seek, unsuccessfully, a cure for his illness.
The return from Marienbad in mid-August marked, however, a turning-point that led Weber to his last major composition, as he found in his mail a letter from Charles Kemble, manager of the Covent Garden theatre in London. The success of Der Freischütz in Germany had prompted informal overtures from London and Paris in 1822 and 1823, and during Weber's trip to Vienna in autumn 1823 Barbaia had tried to interest him in a commission for Italy. But when a veritable mania for Der Freischütz swept London in 1824, Kemble decided to make a firm offer to the man of the hour, inviting him to London to conduct the opera season and compose two new operas for Covent Garden. Recognizing that his days were numbered and believing that the potential rewards offered by London would provide a measure of financial security for his family (which was soon to expand with the birth of a second son, Alexander, on 6 January 1825), Weber wrote to Kemble on 7 October to agree to compose an opera based on Wieland's romance Oberon and to go to London for three months in spring 1825. To prepare himself for composition to an English libretto and the trip to London, Weber – who had earlier learnt some Czech during his stay in Prague and had taken Italian lessons during his first year in Dresden – began to study English, and by January 1825 he was able, with the help of his friend C.A. Böttiger, to correspond with his British contacts in their native language.
Weber received J.R. Planché's libretto for Oberon in three mailings between 30 December 1824 and 1 February 1825, by which time he had requested a postponement of the première until spring 1826. An initial reading of the text revealed to him the profound differences between ‘English opera’ and continental opera, which he lucidly analysed in a pair of letters to Planché. While working on Oberon in early 1825 Weber also accepted a commission from George Thomson of Edinburgh to arrange ten Scottish melodies for voice, flute, violin, cello and piano. By early March he was again severely ill and on 3 July he set out for Bad Ems for another cure. There Kemble and the conductor George Smart met the composer on 10 August to discuss his fee, but they were unable to agree terms. Only in early December did Weber accede to Kemble's offer of £500 for the score and British rights to publication, far less than he had hoped for; he subsequently agreed to conduct the first 12 performances for £255 and four (in the event, five) so-called Oratorio Concerts at £25 each.
Although the treatment at Ems produced no physical improvement, Weber returned to Dresden on 1 September in higher spirits than for a long time, and thus he was able to work on Oberon more or less continuously throughout the last part of 1825 and the first months of 1826. The latter months of 1825 witnessed a luxuriant Dresden production of Spontini's Olimpia in November in conjunction with yet another royal wedding, and the long-delayed production of Euryanthe in Berlin in December 1825, for which Weber supplied additional ballet music. Despite shortness of breath and hoarseness that alarmed his friends as to the seriousness of his condition, Weber went to Berlin to supervise the final rehearsals and conduct the first two performances, on 23 and 28 December. Returning to Dresden on 31 December, he celebrated his last New Year's Day with members of the Liederkreis.
In his final months in Dresden Weber's irritation over the mostly unauthorized dissemination of his work abroad led him to try to exert more control over his intellectual property. The actions of the Parisian arranger Castil-Blaze caused special concern. Although he could do nothing about Castil-Blaze's adaptation of Der Freischütz as Robin des bois at the Théâtre de l'Odéon (7 December 1824), Weber wrote in October 1825 to reproach him for the unauthorized publication of the full score of his version. And on learning that Castil-Blaze planned to produce a new work based in part on the music of Euryanthe, Weber wrote again in January 1826, this time threatening to make his protest public, which he did after Castil-Blaze proceeded anyway to incorporate pieces from Euryanthe in the pasticcio La forêt de Sénart (14 January 1826). Chastened by his experiences, Weber took forceful measures to impede the unauthorized distribution of Oberon, sending to the German theatres a circular to remind them of his exclusive right to sell the score and writing letters to German sovereigns requesting privileges prohibiting the publication or sale of unauthorized editions and arrangements within their territories.
Accompanied by the flautist A.B. Fürstenau, Weber left Dresden on 16 February 1826 to begin the long trip to London. On 25 February he arrived in Paris, where he spent a week making contacts with prominent musicians and poets, including Auber, Berton, Catel, Cherubini, Fétis, Kalkbrenner, Onslow, Paer, Rossini and Pixis. With plans to return to Paris on the way back from London, Weber continued on 2 March, crossing the channel from Calais to Dover on 4 March and reaching London the next day. Staying at the home of George Smart in Great Portland Street, Weber completed the overture to Oberon, finished the music to Act 3, and composed two additional pieces specifically for the leading tenor, John Braham. During the month of rehearsals leading up to the première Weber somehow found the energy also to complete the piano score, conduct four Oratorio Concerts at Covent Garden, conduct at the Philharmonic Society, take part in two benefit concerts and perform in a number of aristocratic salons. The première of Oberon on 12 April 1826 was a great success, with lavish settings and spectacular scenic effects that impressed even Weber, and the opera remained popular throughout the season.
With the Oberonperformances behind him, Weber conducted once more at the Oratorio Concerts, participated in three more benefit concerts and made preparations for his own concert at the Argyll Rooms on 26 May, for which he wrote his last composition, a song for the soprano Catherine Stephens (j308). The concert, which also presented the Jubel-Cantate in an English version entitled The Festival of Peace, netted only £96 11s. 0d. In May plans were broached to mount Der Freischütz at Covent Garden on 5 June as a benefit for Weber, but the composer's health ultimately did not permit this. Following his last public appearance at a concert given by Mary Anne Paton on 30 May, he decided to forgo the Freischütz production in order to leave London on 6 June. But he did not live to see his homeland again, as he died alone and quietly in his room at Smart's during the night of 4–5 June, succumbing at last to tuberculosis. To the strains of the Requiem by his beloved Mozart, Weber's funeral service was held on 21 June at the Roman Catholic chapel of St Mary Moorfields, and there his mortal remains were interred. In 1844, thanks in part to the efforts of Wagner, his coffin was transferred to its final resting-place in Dresden.