Speaking ahead of a debate on the problem in Edinburgh tonight, Professor Tom Devine said religious bigotry went beyond football, and he dismissed the idea sectarianism was peddled by only a small number of supporters.
Sectarian songs were sung by a significant number of fans and were an example of underlying religious hostility that stretched beyond the west coast, he said.
Prof Devine said SNP ministers "took their foot off the pedal" on sectarianism and were facing the most "explicit" and "extreme" religious bigotry in decades.
But other academics insist sectarianism is not widespread throughout Scotland, despite a football season blighted by problems on and off the field involving mainly the Old Firm of Celtic and Rangers.
Prof Devine said: "It is clear, and I get the vibrations from the top of the SNP, that they took their foot of the pedal.
"Such is the scale of what has happened, and such is the international reaction of horror, that any government would have to take further action, and that is what is going to happen.
"I cannot recall in my lifetime a situation where sectarianism has been so explicit, extreme and attracted such international attention. It has certainly put Scotland on the map in a rather unsavoury way."
Last week the Scottish Government announced plans for new legislation, which could see people causing sectarian disruption jailed for up to five years.
Last week, official statistics showed a 9.7 per cent increase in criminal charges aggravated by religious prejudice in 2010-11 compared with 2006-7.
Speaking ahead of the debate at Edinburgh University, Prof Devine said the idea of a "small minority" singing sectarian songs was "rubbish", adding that it indicated a deep-rooted problem in Scottish society.
Rangers and Celtic have previously claimed that only a limited number of their fans sing "unacceptable" songs. But Prof Devine said: "These individuals who behave like this at games, and there are many thousands of them, must reflect deeper attitudes of society. The idea of a small minority is just a joke."
He also attacked the police's handling of sectarian singing at football matches, saying the powers they have are not being properly used.
"It goes back to the notorious sectarian singing at the Scottish League Cup final, where collectively several thousand people were brazenly carrying out a sectarian breach of the peace and the police did nothing", he said.
"I cannot understand this statement which they make, 'We cannot arrest ourselves out of this situation'. It is almost to say that the only way to deal with this is by indirect means. But what you do is make sure that whenever that horror gets into the public domain, there is an effective and rigorous implementation of the law.
"The laws are there - it is just that people are being allowed to get away with it."
However, Dr Michael Rosie, another leading academic on the subject of bigotry, said Scotland was "not a sectarian society", arguing that the problem centred on football.
"These problems arose because of unusually high tensions on the pitch, or, more accurately, tensions on the pitch between Celtic and Rangers," he said.
"The relationship between the two clubs who utterly dominate Scottish football is at the very heart of the problem. It focuses, shapes, provokes and timetables religious bigotry."
Among those expected to attend tonight's debate are former Celtic director Michael Kelly, the historian Owen Dudley Edwards and Glasgow MSP Bob Doris.