No 10 has declined to say how Theresa May will judge if Scots heed her call to use the local elections to voice opposition to another independence referendum.

At the weekend the Prime Minister said that the May 4 vote was a chance to send a “clear message to the SNP” by voting Conservative.

But No 10 refused to say what level of support for her party would suggest that Scots had rejected a second vote.

Downing Street also declined to comment on reports that Whitehall is preparing for another referendum amid speculation Nicola Sturgeon could call for one at her party conference next month.

Asked if Mrs May would veto the vote, a spokesman said that he would not comment on “hypotheticals”.

Mrs May faces a dilemma if the SNP do call for another referendum.

Technically the legal right to hold one lies with Westminster.

David Cameron temporarily gave Holyrood the power to hold the 2014 vote.

Mrs May will be under pressure to follow suit, or risk a spike in support for independence if she blocks another poll.

In recent weeks No 10 has highlighted opinion polls which suggest that most Scots do not want another vote any time soon.

Yesterday a No 10 spokesman said: “The question is not whether there could be a second referendum but whether there should there be a second referendum and the answer to that is no. There was a vote in 2014 the people of Scotland made a decision then to remain in the Union and all the evidence around at the moment suggests that Scotland does not want another referendum. There was a fair, legal and decisive result and both sides agreed they would abide by the result.”

Asked how Mrs May would judge if voters had rejected another independence referendum in May’s election, he said: “I’m not going to second guess the outcome of an election”.

But No 10 did not deny that preparations were being made for a second referendum, saying “not going to comment on anonymous sources”

Scottish Labour’s Westminster spokesman Ian Murray said that the Union was “not safe” in Mrs May’s hands.

“The Tories have consistently put the Union at risk with their reckless actions in government, including the drive for a hard Brexit and the decision to introduce English Votes for English Laws,” he said.

Mrs May's local election call came in Holyrood magazine ahead of this weekend’s Scottish Tory conference in Glasgow.

Ms Sturgeon has said that Brexit makes another vote on Scotland's future “highly likely”.