Deputy First Minister Nicola Sturgeon and Scots Labour leader Johann Lamont have clashed angrily over the SNP's currency plans during STV's latest live referendum debate.

In a heated but ultimately inconclusive contest, Ms Sturgeon refused repeatedly to be drawn on the Scottish Government's Plan B for the currency, repeating her claim that the UK would agree to share the pound in a monetary union with an independent Scotland, despite Chancellor George Osborne ruling out the proposal.

As Scotland's two most senior women politicians cross-examined each other in the hour-long Scotland Tonight Referendum Special, she was asked by Ms Lamont why she believed the Chancellor would change from "a bully to Santa" after the referendum.

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In the sharpest encounter of the night, Ms Lamont asked: "Do you know what the word 'no' means?

Ms Sturgeon replied: "In September we'll know what what the word 'yes' means."

The currency provoked the fiercest clashes in an often angry debate, during which presenter Rona Dougall stepped in repeatedly - but sometimes unsuccessfully - to prevent the two MSPs from talking over each other.

Under questioning from Ms Sturgeon, Ms Lamont denied that independence was required to block unpopular Westminster welfare reforms or remove Trident nuclear weapons from the Clyde.

She said: "I've grave reservations about the renewal of Trident but that debate has to be won and we have to co-operate round the world on making the world a safer place."

She accused the SNP of using the Clyde-based nuclear deterrent as a "bargaining chip" in its campaign for independence.

Ms Sturgeon also challenged Ms Lamont on "the risk of a Tory government" if Scots voted No in the referendum and quizzed her on pensions, saying it was unfair that Scots received less under the Westminster system because of the country's lower life expectancy.

Ms Lamont said she was "astonished" the SNP was basing a policy on Scots dying younger.

Earlier in the debate, Ms Lamont refused to be drawn on Scottish Labour's plans for further devolution, saying they would be outlined in the party's conference next month.

During her opportunity to question the Deputy First Minister, the Scots Labour leader challenged her rival on the potential loss of shipyard jobs in an independent Scotland.

"This is surreal," said Ms Sturgeon, "Govan is being closed under the Westminster system."

As in STV's previous debates, the first half comprised a conventional debate moderated by Rona Dougall.

In the second half of the hour-long show, Ms Sturgeon and Ms Lamont were each given time in which to cross examine their rival.

The two - either of whom could become First Minister in 2016 - avoided gaffes but failed even to come close to landing a killer blow on their opponent.

The debate was the fourth in STV's Scotland Tonight Referendum Special series. Ms Sturgeon has taken part in each of the televised head-to-heads.

She came out comfortably on top, most observers agreed, against former Scottish Secretary Michael Moore and his successor Alistair Carmichael.

Between those debates was a notably bad-tempered clash with Scottish Labour deputy leader Anas Sarwar, after which both sides claimed victory.