Drinks producers in Scotland had until the day before, February 28, to register with the scheme's administrator Circularity Scotland Ltd (CSL) and its regulator SEPA, if they still wanted to trade after the scheme goes live on August 16.
After the deadline Ms Slater, the Minister for Circular Economy, could tell the Scottish Parliament how many had signed up to the scheme (664 she said) and how many had not - a figure she repeatedly refused to reveal, leaving opposition Conservative MSPs to make their own calculations.
The Lothian MSP announced to the chamber on Wednesday: "As of this morning, 664 drinks producers, representing over 90 per cent of the total volume of drinks containers sold in Scotland each year have completed registration.
"There are still concerns among producers, and particularly the smallest companies affected by the scheme. I have consistently said that SEPA will take a flexible approach where possible. Producers can continue to register even now after yesterday's deadline.
"Today sees us move to the next stage of registration, with Circularity Scotland's announcement of the launch of registration for return point operators: the corners shops, supermarkets, and other outlets where customers can return empty containers, and reclaim their deposit.
"So many countries have operated similar schemes for years. The EU is requiring all their members to have one in place by 2029.
"Our scheme will reduce littering by a third, and increase recycling rates of single use drinks containers towards 90 per cent, and will reduce CO2 emissions by four million tonnes over 25 years, the equivalent of taking 83,000 cars off the road.
"This scheme will go live in August, and I can say that with confidence, because people have voted with their feet. Producers have registered, and I am both grateful to them and proud that Scottish businesses are rising to the challenge.
"Business is on board. This scheme will create over 500 jobs. DRS will see over £300 million invested by the private sector in ending the blight of litter on our streets. If you want to prove you are in favour of business, investment, and jobs, get behind the scheme now."
The Shadow Cabinet Secretary for Economy, Fair Work and Culture, Tory MSP Maurice Golden, said: "Small producers are appalled by the scheme roll-out and have not registered, which is why the Scottish Government has been forced consider introducing a grace period.
"It's too late now to make such fundamental changes to the scheme, without creating even more complexity, confusion, and cost.
"And now registration is to remain open - so much for the deadline that effectively forced some producers to sign up, feeling a gun was at their head because they would be unable to trade otherwise. The bottom line is that producer registration has been a disaster.
"The minister is deperately spinning the sign-up numbers, but the reality is it works out to barely 16 per cent of total number of producers. 664 out of 4,000 operating in Scotland.
"And no wonder. Legal advice taken by a number of companies was not to. Why sign a blank cheque to CSL at this point? Does the minister accept that more than 80 per cent of producers have not signed up to the scheme?"
Ms Slater replied: "As of today, producers responsible for more than two billion drinks containers have signed up with Circularity Scotland, and this represents the full range of drinks producers, from global brands to small craft breweries and distilleries.
"This means that more than 90 per cent of the annual total volume of products are included in the scheme: 90 per cent of what we see on our shelves. This means that the scheme has momentum towards that August 16 launch."
More MSPs asked Ms Slater for the total number of producers in Scotland, or the percentage that had signed up to the scheme, including Labour MSPs Colin Smyth and Daniel Johnson, and Conservative MSPs Douglas Ross and Craig Hoy, who ventured that "84 per cent of producers have not signed up to the disasterous scheme".
Ms Slater said the question was based on a "misunderstanding" of "what makes the scheme viable and builds momentum".
Eventually Mr Ross, leader of the Scottish Conservatives, said: "Either the minister and Scottish Government do not know that figure, or the minister has refused repeatedly in the chamber this afternoon to give that figure."
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