On seeing the quality of the notes, one bank official described them as "the most dangerous ever seen"; the watermark was the most reliable source for detecting the forgeries. Counterfeit notes worth £15–20 million were in general circulation at the end of the war. With such a volume in general circulation, in April 1943 the Bank of England stopped releasing all notes of £10 and above. In February 1957 a new £5 banknote was issued; the blue note was printed on both sides and "relied on subtle colour changes and detailed machine engraving" for security. Other denominations were also reintroduced: the £10 in February 1964, the £20 in July 1970 and the £50 note in March 1981.