Two bombs exploded: the first in the Mulberry Bush at 8.17pm; the second in the Tavern in the Town 10 minutes later.
Twenty-one people died, 182 were injured. It was the biggest terrorist death toll in Britain until the 2005 London bombings.
The bomb warning to the Birmingham Mail came at 8.11pm, using the recognised IRA codeword of the time, “Double X”.
The caller, who had an Irish accent, said a bomb had been planted “at the tax office’ on New Street – offices that were above the Mulberry Bush.
The Birmingham Six were Hugh Callaghan, Patrick Joseph Hill, Gerard Hunter, Richard McIlkenny, William Power and John Walker.
The Birmingham Six were found guilty of the bombings in 1975. Each received 21 life sentences. The judge said they had been convicted on the “clearest and most overwhelming evidence that I have heard”.
The court of appeal rejected this in March 1991, quashing the convictions as “unsafe and satisfactory”.
One of the Birmingham Six families had to move home 17 times because of death threats made to them.
