Labour last night insisted they would not make any concessions to the SNP after it emerged the party's one seat advantage at Holyrood would vanish when one of their MSPs goes on maternity leave.

It was revealed yesterday that Angela Constance, the new Nationalist MSP for Livingston, was expecting her first child in the autumn. Her absence on maternity leave will mean the SNP and Labour will each have 46 seats.

First Minister Alex Salmond yesterday appeared to challenge Labour not to use Ms Constance's pregnancy to block SNP legislation by insisting no party would "stoop so low" as to seek political advantage out of the situation.

"I am quite certain that whatever differences we have with the other political parties, none of them would want to take advantage of Angela's maternity leave to cause difficulties for our administration. I don't really think politicians think like that."

Mr Salmond added: "I don't think our political opponents will stoop that low. If there are difficulties we will work our way through them. As a government which has to command a majority by persuasion and argument, we will just have to persuade and argue all the more forcibly."

But a spokeswoman for the Labour Party said Ms Constance's maternity leave would not influence how they conducted themselves in the parliament.

"The Scottish Labour group wishes Angela Constance all the best during her pregnancy and maternity leave," she said. "Angela's absence during her leave will in no way influence Scottish Labour's priorities for opposition, or our voting intentions on the issues that come before parliament."

The genesis of the dispute dates back to last year when the SNP withdrew from a parliamentary arrangement known as pairing''. Under the agreement, parties withdraw one of their MSPs from voting if a member on the opposition benches is absent.

A Labour Party source said: "We were all in favour of pairing until they ditched it."

Ms Constance, who defeated Labour's Bristow Muldoon to take the Livingston seat, said she and her husband Garry Knox were very pleased.

She said: "Many parents have obligations at work and home and we will be no different. I look forward to the exciting challenges that lie ahead as a new mum and new MSP."