Scottish Conservatives leader Ruth Davidson has set out her opposition to calls to introduce a quota to ensure a 50/50 gender balance in Holyrood.

Ms Davidson said quotas are a crude tool that by definition could not deliver true equality.

A cross-party group of MSPs, including from Labour, the Liberal Democrats and the Greens, is pushi ng for the move as part of calls for new powers to be awarded to Holyrood.

The Tory MSP said: "I have always been against gender quotas, whether for business, politics or academia. They are a crude tool which, by their very nature, cannot deliver true equality.

"Many women count themselves out before they ever get to a selection or recruitment process and we have to address why that is.

"There are much better ways to reduce under-representation, such as through mentoring and support.

"For many people, women who achieve their position through a quota system will always - and often quite unfairly - have a question mark hanging over them."

By November the leaders of the three largest parties at Holyrood are all expected to be female. As the favourite to succeed Alex Salmond at the top of the SNP, Nicola Sturgeon is tipped to join Labour's Johann Lamont and Ms Davidson at the top.

The cross-party group is pushing for the gender issue to be addressed in all parties manifestos in the run-up to the 2016 Holyrood elections.

Among the Scottish Conservatives' MSP group 40 per cent are women. As leader, Ms Davidson has established the post of vice-chairman for women, designed to help identify and support potential female candidates.

Ms Davidson's comments came as she hit out at criticism of her position in a police investigation into allegations pro-Union campaigners breached electoral secrecy laws by counting postal votes ahead of referendum polling day. In the wake of assertions on micro-blogging site Twitter, she defended her part in the inquiry, calling for one tweet to be retracted. It is understood it was.

The development comes as the Crown Office instructed a police investigation into alleged breaches. It followed an assessment of complaints surrounding comments made by Ms Davidson who said on a televised discussion that postal vote "tallies" were being taken in the weeks before the referendum ballot closed at 10pm on September 18.

Police made initial telephone contact with Ms Davidson on Friday, a week after the force began an assessment of complaints. Party sources said there was "no suggestion she was accused of doing anything wrong".

On televised coverage of the referendum results, 45 minutes after the polls closed, Ms Davidson said the No camp had been "incredibly encouraged" by the results of a "sample opening" of the postal ballot she said had taken place around the country over the few weeks prior to the poll. Complaints over her account of the postal vote "tallies" raised concerns the information may have helped inform the No campaign's decision to issue the "vow" of more powers for Scotland from the three main party leaders.

In what was believed to be a reference to The Herald she added in another tweet: "As the piece makes clear there is no suggestion I have done anything wrong."

The Electoral Commission passed complaints to the police after making an initial assessment over whether there was any case to answer. Political agents and campaigners are allowed to oversee the postal vote opening sessions, where checks are made to verify the signatures and dates of birth on postal voting statements against computerised records.