Plans to give more powers to Wales are so flawed AMs should reject them altogether, a team of academics and former senior civil servants has said.

More say over energy, transport and the assembly's own elections are on offer.

But a report said the draft Wales Bill was "constricting" the assembly, by the way it was reserving many matters as the responsibility of Westminster.

Welsh Secretary Stephen Crabb said he was open to changes amid accusations the bill may reduce Welsh power.

It proposes moving to a "reserved powers" model for Wales, by which all policy areas are assumed to be devolved unless specified as remaining with the UK government.

Opposition parties have claimed the list of reservations is too long.

Monday's report from Cardiff University's Wales Governance Centre and University College London (UCL) said the UK government's latest devolution proposals were "clunky and short-sighted".

'Triple squeeze'

Professor Richard Rawlings from UCL said Wales had experienced "three deeply problematic devolution settlements" since 1999, and there had been hopes of a process putting Welsh devolution on a "sustainable constitutional basis".

But he warned: "The draft Wales Bill does not do what was promised.

"All too often, the Secretary of State's fine policy objectives of a stronger, clearer, fairer and more robust devolution settlement are frustrated by provision that is constricting, clunky, inequitable and constitutionally short-sighted.

Prof Rawlings said the assembly faced a "triple squeeze" of "intrusive general restriction, over-occupation of legislative space, and blurry forms of executive veto".

MPs on the Welsh Affairs Committee have been carrying out their own inquiry and are expected to publish a report soon.

A spokesman for the Wales Office said: "We want the Wales Bill to create a stronger Wales within a strong United Kingdom and we published a draft bill so that we could listen to comments and make sure we get this right.

"Clearly before publishing the full bill we have to wait for the Welsh Affairs Committee to publish its report, and then give due consideration to its recommendations."

A Welsh government spokesperson said Monday's report backed the first minister's assertion that the draft Bill was "too important to be rushed".

"We urge the UK government to delay introduction of the Bill and take the time needed to get this right, with proper engagement between UK and Welsh governments and other stakeholders," the spokesperson added.

Plaid Cymru leader Leanne Wood said Monday's report echoed many of her party's concerns.

"The only way of securing a robust and sustainable devolution settlement is to give Wales parity with the other nations of the UK," she said.

"At present, Scotland and Northern Ireland are being handed significant tax powers while English cities will gain control over policing - all of this without a referendum.

"Time and again, Wales is forced to jump through more hoops than any other part of the UK, only to end up with less control over our own affairs."

Analysis by Nick Servini, BBC Wales political editor

By the standards of the usual dry constitutional language associated with reports like this, there was some eye-catching criticism here.

The authors say the draft Wales bill, if left unchanged, would give lawyers a field day and have a "chilling effect" on AMs who would be reluctant to legislate because of the lack of certainty on the boundaries.

We are told Stephen Crabb is in listening mode. He will need to be in reading mode as well, with another report from Welsh MPs on its way on the same subject.

The Welsh secretary has already said he is going to make alterations. The question is whether there is enough time available to him to make the kind of changes that are being called for.