The convertibility of the banknote was suspended at the start of World War I.
At a time when the public was hoarding metal coins, the National Bank took the emergency measure of printing a "Current account" series, which even included small denominations of 1 F and 2 F. Royal portraits appeared on the notes for the first time. To punish the National Bank for evacuating its stock of banknotes and gold reserves to London, the occupying power suspended its right of issue and conferred that right on the Société générale: thus, at that time, banknotes were issued in the latter’s name. To deal with the currency circulation problems, over 800 municipalities in turn decided to issue "emergency currency". Finally, the German mark also became legal tender and flooded the Belgian economy. When the war ended, these marks were exchanged for Belgian francs at a favourable rate, in the expectation of German war reparations which were only partially honoured. Between 1914 and 1918, banknote circulation doubled while the economy was in ruins: inflation took hold.
In 1926, a new unit of account, the belga, was intended to dissociate the Belgian franc from the French franc and improve the convertibility of the Belgian currency, which was in dire straits. Its value stabilised at only one-seventh of the former parity.
All these movements were reflected to some degree in the banknotes and coins of the day. Immediately after the war, when 1 F coins were minted for the first time in a non-precious metal nickel – they were marked with the words "good for", to indicate their intermediate function. The word "belga" is found on banknotes and coins between 1926 and 1944; never on its own, but always accompanied by the word "franc". Immediately after the war, a surge of patriotism prompted Belgium’s central bank to show a portrait of a reigning sovereign, King Albert – and his wife – for the first time on Belgian banknotes belonging to the "national series". In 1929, persistent inflation led to the issue of a 10 000 franc - 2 000 belga banknote, the largest denomination ever issued in Belgian francs.
In 1921, Belgium concluded a treaty on economic union with Luxembourg, facilitating the circulation of Belgian banknotes in that country. However, it was not until 1935 that Belgian banknotes became legal tender there. In the same year, a further devaluation, the third since the war, caused a 28 % reduction in the value of the Belgian currency. That is why the commemorative silver coin issued in 1935 was placed in circulation at a face value of 50 francs, instead of 40 francs as planned, and as shown on a small number of sample coins.
The last pre-war banknote was issued in 1933: it shows the portraits of King Albert I and Queen Elisabeth, plus a symbolic representation of the Meuse, the Escaut and the Albert canal, construction of which helped to absorb excessive unemployment.
World War II brought a new batch of disasters. For the man in the street, the situation grew worse from 1942 onwards. Wages were frozen and prices escalated, increasing by up to 650% on the black market. Between 1940 and 1944, the quantity of cash in circulation tripled and the economy was in ruins. From June 1940 the occupying power imposed the use of the German currency in parallel with the Belgian currency, and from 1941 the coins in circulation were replaced by war currency made of zinc, as in World War I.
The National Bank, whose directors were in exile in England, evacuated its banknotes and reserves abroad. As in World War I, the German authorities retaliated by appointing a new issuer, called the Brussels Bank of Issue. The banknotes which the National Bank was obliged to print on its behalf were never placed in circulation.
However, in London the Belgian government in exile – funded partly by the Belgian gold owned by the National Bank, and recognised by Churchill, prepared for the post-war period. Camille Gutt, the Finance Minister, was firmly resolved to prevent a recurrence of the inflation which had afflicted Belgium after World War I. As soon as Brussels was liberated, in October 1944, he set up a massive operation for the withdrawal of the existing banknotes from circulation, exchanging them up to a certain maximum value; this was known as the Gutt Operation. For this purpose, the British company of Bradbury printed banknotes in denominations of 1,000, 500 and 100 francs, a small-scale version of the pre-war Emile Vloors banknote. 10 and 5 franc notes with a simple guilloche design were printed by the De La Rue press. They were used mainly by the Allies as a means of payment at the time of the Liberation. Outside the National Bank there were long queues of people waiting to exchange banknotes, but on completion of the operation there were only 57 billion francs left in circulation compared to 165 billion at the start. This opened the way to reconstruction.
1945-2002: the rise and the disappearance of the Belgian franc