SNP leader Nicola Sturgeon was booed on Tuesday night when she refused to rule out a second independence referendum. She told the audience at the Scottish leaders’ debate that while independence was not an issue at the general election, offering another vote as part of the 2016 Scottish Parliamentary elections manifesto was “another matter.”

She said: “This Westminster election is not a re-run of the referendum campaign. If you vote SNP in this election it doesn’t mean Scotland has another referendum, or becomes independent as a result of that.”

But when she was pressed by STV’s political editor, Bernard Ponsonby, about what would happen after 2016 she said: “That is another matter, we will write that manifesto when we get there. I will fight one election at a time.” The comment was followed by groans and boos from the audience.

Before the referendum bill was passed two years ago, Sturgeon said of the planned vote: “We have always said it’s a once in a generation thing.” The nationalist leader also confirmed she would be willing to prop up Ed Miliband but would never do a deal with the Tories. This led Scottish Labour Leader, Jim Murphy, to say: “Nicola, we don’t need your help.”

The SNP has been keen to tell the public that they would prop up a Labour government. This is because Labour had suggested that a vote for the SNP would help the Conservatives by making it harder for Ed Miliband to form a majority.

Party strategists within each of the parties believe that Scots are much less likely to vote SNP if they believe it will help the Conservatives. Polling data also shows that Labour voters who have switched to the SNP may well come back if they think supporting Sturgeon risks keeping David Cameron in power.

In September last year, 55 percent of the Scottish public voted against independence. This was despite the SNP being given a number of advantages, most notably that 800,000 Scots living in the rest of the UK were not allowed to vote. However, Scots living outside of the UK were allowed to join the 3.6m in the country that voted.

It is unlikely the Westminster Parliament would willingly pass a bill to allow a second Scottish referendum. But if the SNP end up in coalition with Labour, the government would be forced to push through a number of unpopular measures, such as the abolition of the Trident nuclear deterrent.

SNP plans for Westminster are so distrusted amongst English, Welsh and Northern Irish voters that the Conservatives released this attack ad about Alex Salmond: