He said there were two broad possible explanations: a strong last-minute swing to the Conservatives, or polling problems.
"There is a track record here. The truth is that at most though not all recent elections, the polls have tended to underestimate the Conservatives and overestimate Labour."
Some pollsters admitted something had gone badly wrong, and they did not yet understand what.
"Election results raise serious issues for all pollsters," said Populus, one of the main polling firms, on Twitter.
Others, such as Survation, ComRes and Ipsos MORI, defended themselves, saying they had been right about the Scottish National Party's surge, the collapse of the Liberal Democrats and a sharp increase in vote share for anti-EU party UKIP.
Andrew Hawkins, chairman of ComRes, said this fragmentation of the political landscape had presented pollsters with "extra headaches" by turning the election into a "patchwork of regional contests" where national trends were less relevant.