Scotland's deposit return scheme should have the scope to bring in a "variable rate" on charges to be levied on drinks cans and containers, MSPs have said.

While members of a Holyrood committee said they were "content" with the proposed 20p deposit, they suggested the scheme's administrator should have the scope to vary this - with larger bottles potentially incurring a higher charge.

They made the call after hearing how the fixed rate 20p deposit - which shoppers will get back when they return their empty containers - could have unintended consequences such as prompting people to buy larger sizes of some drinks.

In a new report on the scheme, being brought in by the Scottish Government as part of efforts to increase recycling, they noted the "potential unintended consequences on plastics use and health impacts".

They said: "The committee considers that there should be scope for the scheme administrator to set a variable rate, for example, based on product size.

"Variable rates could be used in a number of ways to discourage materials switching or other unintended consequences, with 20 pence being the minimum level of deposit."

Members of the Environment, Climate Change and Land Reform Committee said the deposit return scheme should be "as comprehensive as possible".

They asked ministers to set out an "indication of the likely time-frame for extension of the scheme".

When introduced, the fee will apply to the majority of drinks containers, including PET plastic bottles - used for fizzy drinks and water bottles - glass bottles and steel/aluminium drinks cans.

Cartons and pouches as well as other types of plastic are not covered, with the committee calling on the Government to clarify "as a matter of priority" if new regulations would be required to add different kinds of containers to the scheme.

MSPs also want to know if the Scottish Government can compel online retailers based outside of Scotland to participate in the initiative.

They suggested the inclusion of online sales could be "critical to the effective implementation of the scheme and to meeting the recycling targets". While there is broad support for the scheme, the committee said ministers should ensure the level of the deposit does not "adversely impact" poorer Scots.

It published its report after the Scottish Government introduced draft regulations for the deposit return scheme to Holyrood.

The committee convener, SNP MSP Gillian Martin, said: "There is no doubting the ambition behind the introduction of the deposit return scheme in Scotland.

"And it is clear from those we heard from across the country that there is support for the scheme and its aims to help meet the challenging climate change targets and address the problems of litter, particularly in our marine environment.

"But for this scheme to be effective, we need to ensure that this operates in a way that is understandable and easy to use for the public and businesses alike."

She added: "We support the initial focus of the scheme but have today asked the Government to ensure we can add materials to the scheme, in a phased way, so it is as wide-ranging as possible.

"We have also heard from many businesses about their concerns about how this will operate in practice.

"We believe the Scottish Government needs to listen to these concerns and we have asked for more information on the operation of the scheme before this is rolled out."

Responding to the report, a Scottish Government spokesman said: "We welcome the Environment, Climate Change and Land Reform Committee's report on the draft regulations for our ambitious deposit return scheme and note its overall support for the introduction of the scheme, which will help tackle climate change, reduce litter and deliver a more circular economy.

"We are pleased the committee agrees the scheme should be as comprehensive and accessible as possible, enabling people to do the right thing by recycling their metal cans, glass and plastic bottles.

"We will carefully consider the report and will continue to work closely with our implementation advisory group, and directly with stakeholders, as we work to finalise the regulations."