Scottish Labour's Monica Lennon wants to introduce a legal right of access to free products such as tampons and sanitary pads, an "ambitious" move she says would make Scotland a world leader in tackling "period poverty".
Her Bill would put a statutory duty on councils and education institutions to supply period products, but today in giving evidence to a Holyrood committee, Ms Lennon was quizzed on costs which she has put at £9.7 million a year but the Scottish Government claims could be £24 million annually.
The government has said it will not support her Bill, as it believes a targeted approach is more cost-effective than universal provision.
Today, Tory MSP Graham Simpson said: "The nub of this is we actually have no idea what this would cost if this legislation was passed. Nobody has a clue.
"The figures you have provided are way different to those of the Scottish Government, so I don't know who is right and who is wrong."
The Labour MSP said she had set out "costings in detail", based on figures drawn from Scottish Government information. But she said: "In contrast the Scottish Government has not set out workings for how its higher estimated costs have been arrived at."
SNP MSP Annabelle Ewing claimed the plans would be "much, much more expensive than the approach the Scottish Government is taking" and questioned what would have to be cut from the Scottish Budget to pay for the scheme, if the legislation is passed.
But Ms Lennon said the move could be seen as an "investment in women and girls".
She said: "We have heard about the cost of not doing this, about the people who find themselves in period poverty, the people who can't experience period dignity. I think there is an opportunity in doing this, we've heard about the benefits in terms of productivity, about participation, about quality of life, about the benefits to education.
"This is an investment, an investment in our young people, an investment in women. It's about making sure women and girls feel valued."
Ms Lennon said the costs could be resolved and she was due to meet with the government's Communities Secretary Aileen Campbell to discuss the issue, but said MSPs should back her Bill as it was the "right thing to do".
She said Holyrood should "send a clear message that this Parliament takes period dignity and access to period products seriously" and added: "It is ambitious because we would be the first country in the world to take this step, but it is the right thing to do."
She said the approach so far adopted by the government, targeting schemes at low-income women, meant some risk missing out. "For those people who can't afford it, this universal scheme will be a lifeline. That is fundamentally what this Bill is about."
The Scottish Government already funds free period products in schools, universities and colleges across Scotland, potentially helping almost 400,000 students. In addition, more than 35,000 women who are deemed as being most in need benefit from free products, with ministers also providing £2.8 million to ensure pads and tampons are made available in buildings such as libraries, community centres, council offices and public toilets.
Aileen Campbell has already told the committee the Bill is not needed. She previously told MSPs: "We do believe we are beginning to see a culture change and there is a risk that introducing legislation now would encourage people to meet only minimum standard, compared to what we are seeing in the public, private and third sectors, where organisations are going above and beyond."
However Ms Lennon's Bill has been backed by dozens of charities, organisations and campaign groups who today signed an open letter of support.
In the letter they welcome the steps already taken by the government but added: "To build on this progress, we are writing to ask for your support for the Period Products (Free Provision) (Scotland) Bill.
"We believe that period poverty remains a significant issue in Scotland, and that access to free period products for all who need them should become a legal right.
"The passage of the Period Products (Free Provision) (Scotland) Bill is an important step towards normalising menstruation and helping to end the stigma around periods."
It goes on: "We believe that creating a statutory duty is necessary to ensure that this progress continues and that future governments cannot undo the work that has already been undertaken.
"The Bill has wide support across Parliament and civic Scotland, and we urge you to support the general principles of the legislation so that it can progress to the next stage of further scrutiny and opportunity for amendment."
Other signatories include Aamer Anwar, lawyer and Rector of the University of Glasgow and Amika George, the founder of the worldwide movement Free Periods.