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The Welsh assembly's presiding officer has called for the number of AMs to be increased from 60 to 80.

Rosemary Butler makes the call in her submission to the Silk Commission examining whether further powers should be devolved to Wales.

She said an increase in AMs would be part of a "strong and clear constitutional settlement for the assembly and the people of Wales".

The UK government will decide what to include in a Government of Wales Bill.

"Given the weight of responsibility resting with the institution, and the unavoidable scale of the workload faced by members, I am in no doubt that the number of Assembly Members should be increased from 60 to 80," said Ms Butler.

Plaid Cymru leader Leanne Wood said it was an interesting proposition as AMs would be "limited" to deal with all the work when more legislation has to be discussed and processed by the assembly in the years to come.

"What I would be interested to see as well is a corresponding reduction in the number of MPs so we don't end up spending extra money on this," said Ms Wood.

The Electoral Reform Society said having more AMs could save money for the taxpayer because politicians act as watchdogs for government spending.

It said the money saved by cutting the size of the House of Lords could be spent on a bigger assembly - something that was needed to hold the Welsh government to account.

The Welsh Liberal Democrats agreed, saying: "While it is not a popular call to make, 60 assembly members scrutinising and legislating on important issues such as economic development, health, education and other important areas is not enough."

However, the Welsh government has previously said it does not think there is a public appetite for more AMs.

A spokesman said: "The commission may feel that they want to look at the size of the assembly, in the light of the various evidence being submitted on further powers."

The Welsh government's own evidence to Silk calls for the devolution of powers over policing.

Ms Butler also cites a number of recent examples which illustrate what she calls the "unclear and uncertain" boundaries of the current devolution settlement.

The extent of the assembly's powers should be defined "to give us greater legal certainty" so the institution can legislate "effectively and with confidence".

The lawfulness of the first bill passed by the assembly under its new law-making powers was challenged in the Supreme Court by the UK government.

A panel of five judges found in the assembly's favour on every count.

However, Ms Butler said this and other examples illustrated the need for clearer boundaries and more autonomy for the institution.

"The pace of constitutional change in Wales has been dramatic and the institution today is very different to the one envisaged by the UK parliament as recently as 2006," she said.

"There should be a fundamental recognition that the assembly, not the UK parliament, is best placed to determine certain matters for itself and should not be subject to unnecessary restriction."

As part of her submission, she says that in future, it should be described as a parliament, rather than an assembly.

The Silk Commission was set up by the UK government to examine the future of devolution in Wales. It has already reported back on the first stage of its work, recommending that some tax-varying powers should be transferred.

Its second stage is under way, looking at whether powers in more policy areas should be devolved. A move to 80 AMs was recommended by the Richard Commission in 2004, but not implemented.

Campaign group True Wales, which was opposed to further law-making powers during the referendum in 2011, said that a Yes vote was likely to lead to an increase in the number of AMs.

However, it will be up to the UK government to decide what it puts in a new Government of Wales Bill which would be needed for any substantial changes to the assembly's powers or structure, including new members.

It is expected to release its submission to the Silk Commission next week.