Media playback is unsupported on your device Media caption Stephen Crabb warns the economy could be hamstrung by endless debate

Devolution is failing to deliver economically vital projects such as an M4 relief road and rail upgrades, the Welsh secretary has claimed.

Stephen Crabb told Cardiff Business School he was unhappy at the lack of progress, and that opponents would rather argue about assembly powers.

He said he was "totally open to ideas" about improving the draft Wales Bill.

The Welsh government insisted it was focused on increasing prosperity rather than constitutional change.

In a speech on Thursday he said the UK government was committed to devolving more power.

'Never-ending debate'

In a change of approach, the draft Wales bill recommends a reserved powers model, which lists the policy areas still controlled by the UK government with everything else assumed to be devolved.

Opposition figures including First Minister Carwyn Jones have called for more discussion, claiming the plan could leave the assembly with less power than it currently has.

Mr Crabb has denied the charge, and urged politicians to focus more on jobs and prosperity than a "never ending constitutional debate".

He said he had a "dream" of Wales becoming a "beacon of economic success" within the next 20 years.

But he said he sometimes had "a nightmare" of "an ongoing, inward-looking debate about Welsh powers and the constitution, where we still prop up the bottom of the economic league tables within the United Kingdom and where we are still discussing the need for that damn M4 upgrade."

Opposition parties blamed the UK government for holding Wales back.

The Welsh government claimed: "The only constitutional obsessive is Stephen Crabb, who seems to talk about little else at the moment.

"Meanwhile we are concentrating on the things that matter to the people of Wales, such as the economy and increasing prosperity."

Plaid Cymru Treasury spokesman Jonathan Edwards MP said the Welsh economy faced a disadvantage due to "meagre powers" being offered.

"In their lack of ambition for Wales, the Westminster parties are handcuffing the ability of our government to intervene in the Welsh economy," he said.

Liberal Democrat AM Aled Roberts responded on Twitter, saying the next Welsh government "will have years of UK failure to invest in infrastructure to address".