Nationalist campaigners are bringing shame to Scotland by the way in which they "monster" supporters of the United Kingdom, the former chancellor Alistair Darling has said.

As a new poll showed a growth in support for independence, the head of the Better Together campaign accused nationalists of being "consistently negative" about supporters of the UK.

Darling told the Andrew Marr Show on BBC1: "The negativity is largely coming from the nationalists. In the last week alone when anyone speaks out against them they monster them. We had a businessman earlier in the week who said he thought staying in the UK was good for his business – the behaviour towards him was disgraceful. It brought shame on Scotland.

"Equally the CBI was monstered when they appeared before a committee in the Scottish parliament. We are the ones who are being positive about the case for the UK. The nationalists are consistently negative and will consistently do anything to shout down anyone who speaks out against them."

A Panelbase poll published in the Sunday Times showed that the pro-union lead among voters had been cut to six points from more than 24 points last year. The poll found that – once "don't knows" are excluded – 47% intend to vote for independence and 53% intend to vote to remain in the UK. It is the strongest support for independence this year.

Darling was answering criticism that the Better Together campaign is overly negative. His remarks about a businessman criticised by nationalists referred to Keith Cochrane, the chief executive of the Weir Group, who warned of an increase in business costs if Scotland becomes independent.