The group said its overall index fell deeper into negative territory, sliding by three points to -31, well down on the -16 it stood at in April last year.
It added that it was only the third time the index had dropped below -30 in the survey's 37-year history, with the previous two occasions being in early 1990 and mid-2008.
Nick Moon, managing director of GfK NOP Social Research, said: "Coming after six months of stagnant economic growth, this is a significant drop – one that is bad news for the government and bad news for the economy.
"It suggests that the attempts to spur growth in last month's budget have failed to convince the public, and this may well be sorely felt on the already-beleaguered high street.
"It is particularly striking that all five areas of the index fell this month, pointing towards growing gloom as we head into summer.
"These figures must make the possibility of a double dip recession increasingly real."
Brother Can You Spare a Quid?