De László wrote first of his desire to become a naturalised British subject in a letter to his brother Marczi in 1912. Due to pressures of work and loyalty to his native Hungary he did not instruct his solicitor to commence proceedings until 21 June 1914.  He was formally granted British subject status on 29 August 1914 and swore the oath of allegiance on 2 September. War had been declared on 4 August.

During the early years of the war de László contributed generously to war charities and was much in demand painting the portraits of officers on leave from the front. He also donated canvases for fundraising auctions that secured £4,500 for the British Red Cross.

The artist had long supported his family in Hungary with regular payments of money and they wrote to each other regularly. He continued this practice after the outbreak of war with his letters being passed through General Post Office censors. De László also sent five letters through the Dutch diplomatic mail bag at the behest of Adriana van Riemsdijk. John Loudon, her brother, was the Dutch Minister of Foreign Affairs and had given his consent.

De László was first questioned by police regarding payments to his family in February 1915 and warned against doing so. After censors intercepted a letter referring to money transfers on 3 March his post was secretly opened and monitored by Post Office Special Section. The Home Office began building a case against him and considered charging him under the Trading with the Enemy Act but decided instead to caution him.

The artist was visited at his studio on 17 July 1917 by Árpád Horn, an escaped Hungarian officer from Donington Hall prisoner of war camp. The artist gave him £1 for food and sent him away. De László reported the incident to police the following day and Horn was arrested and returned to prison. Failure to report the incident immediately, discovery that letters had been sent uncensored to an enemy nation through the Dutch diplomatic bag and repeated attempts to send money to his Hungarian family led to his arrest on 21 September 1917 under the Defence of the Realm Act and he was imprisoned in Brixton Prison.

 The case was heard before the Advisory Review Committee headed by Justice Sankey at Westminster Hall on 28 September 1917. Sir Luke Fildes, The Earl of Selborne, Sir Arthur Lee and William Lockett Agnew gave evidence in support of de László. The case was adjourned and he was returned to Brixton Prison before being moved on 7 November to Islington Internment Camp. There he found himself interned with approximately seven hundred other foreign nationals some of whom became close friends. One of these was the German industrialist Theodore Bruno Kittel. The Advisory Committee reviewed de László’s case on 20 December but he was denied release based on concerns expressed by MI5.

These pressures on de László and his financial responsibilities led to a complete physical and psychological breakdown. He was admitted to St. Mary’s Hospital, Paddington, and then granted permission by the Home Office in May 1918 to be released to the care of a nursing home in Ladbroke Gardens, Notting Hill. He was kept under strict house arrest but allowed to receive visits from his wife and children. Finally he was allowed to paint in oil again having been restricted to pencil and watercolour while at Brixton and Islington. He painted still lifes, religious subjects and intimate portraits of his wife Lucy and their children.

An amendment to the Nationality and Status of Aliens Bill in July 1918 mandated a review of all naturalisations granted after the beginning of the war with the possibility of revocation. Although this guaranteed de László’s continued internment until his case could be heard he was permitted to live under house arrest with his family at the home of his solicitor, Sir Charles Russell, at Littleworth Corner in Burnham Beeches, Buckinghamshire. This proved to be one was one of de László’s most creative periods as he was free to paint for pure enjoyment rather than dealing with the demands of portrait commissions. His wife Lucy and children were his primary subjects as was the house and its surroundings.

De László’s case was heard by the Naturalisation Revocation Committee, chaired by Justice Salter, Viscount Hambleden and Judge Radcliffe, during a trial that lasted from 23-27 June 1919. All concerns expressed by the Advisory Committee were addressed and the artist Sir John Lavery, the Chancellor of the Exchequer Sir Austen Chamberlain and Mrs van Riemsdyk appeared as character witnesses. The latter came from Holland despite being mortally ill.

The Committee ruled:

No disloyalty or disaffection has been proved … although there have been breaches of the law in regard to the money, and the carriage of letters, they were inadvertent and stopped when discovered; and with regard to the Horn incident, and to the case generally, we are satisfied that there has not been on the part of Mr de László any conduct which would merit or justify the withdrawal from him of the British citizenship which he enjoys.

His name cleared, his reputation restored and his British citizenship confirmed, de László was finally able to resume his life and work.