From Bronstein's obituary from the British newspaper, The Independent:
In 1976, Viktor Korchnoi defected from the Soviet Union during a tournament in Amsterdam and David Bronstein was one of the few top Soviet grandmasters who did not sign an official letter condemning him. For this sin, Bronstein was banned from travelling to tournaments in the West. The ban was only lifted with the advent of perestroika in the mid-Eighties.
Even before these restrictions were imposed, his international appearances had been limited, but Bronstein made two trips to Britain in 1975-76. Both his eccentricity and brilliance were in evidence in his very first game at Teesside against the English grandmaster Raymond Keene. Bronstein thought for some 15 minutes over his first move, then played 1.c4, the English opening, out of respect, as he later explained, to his hosts. He seemed to obtain no advantage whatsoever, but after another long think sacrificed a pawn. Some of the world's finest players, seeing this, shook their heads in sad disbelief, but Bronstein had seen more than any of them. A few moves later his attack crashed home and a new generation suddenly realised that the old man had lost none of his imaginative faculties.
http://www.independent.co.uk/news/obituaries/david-bronstein-427528.html
In his book, "Secret Notes," on page 14, Bronstein writes:
Baturinsky, the head of the Soviet chess administration, once informed me (soon after the defection of Korchnoi [in 1976], whom I had the audacity not to condemn) that ... personal [tournament] invitations that had arrived for me were of no significance to them. He then pointed his finger upward and pompously stated, "Up there they are unhappy with you."