The Scottish Parliament will automatically receive powers "by default" over fishing and farming once the UK leaves the European Union, a parliamentary committee has judged.

A report authored by the House of Lords EU committee has concluded leaving the EU will result in a "significant increase in the powers and responsibilities of the devolved institutions".

The Scottish Parliament has had responsibility for farming, fishing and environmental protection measures since its foundation in 1999.

It jointly administers these areas inside the common European framework set by the EU.

Britain's devolved settlements were created in the 1990s around the pre-existing European structures of government with the assumption of continued membership of the EU and the supremacy of its law.

The committee believes only a change of law setting out what powers are reserved back to Westminster before the UK officially ceases to be in the EU could stop the transfer of responsibilities to Edinburgh, Cardiff and Belfast.

Such a change in the framework of the British constitution would be "complex and politically controversial," it states.

The committee added: "We doubt that either the UK Government or parliament has the capacity to undertake such a task at the same time as achieving a successful Brexit."

In addition to the change in where power lies in Britain, the Lords argue it is time to "bite the bullet" and "replace the Barnett Formula with a needs-based funding arrangement" after the UK leaves the EU.

Commenting on the report, Lord Jay of Ewelme said: "Brexit's impact on the future of the United Kingdom will be profound and unpredictable.

"At the moment the internal politics are pretty toxic and we saw only last week the start of what could become a deep and bitter dispute on the role of the devolved institutions in passing the EU (Withdrawal) Bill.

"We can't afford this. The UK Government must respect the devolved institutions. It's not enough saying it's listening to them - it's actually got to take account of what they say and adjust its approach to accommodate their specific needs.

"Equally the devolved administrations must work with, not against, the UK Government to get the best Brexit for the whole of the UK."

In December, the Scottish Government called on the UK Government to allow Scotland to remain inside the European single market even if the rest of the country leaves the trading bloc.

The UK Government rejected the proposal and the committee described the plan as "politically impracticable, legally highly complex and economically potentially disruptive to the functioning of the UK single market."

The committee called on the UK Government "raise its game" to find better use of the joint ministerial committee (JMC), which brings together central government and the devolved administrations.

During the course of the negotiations the Brexit subcommittee of the JMC "should be authorised to agree common positions on key matters affecting devolved competences in time to inform the UK Government's negotiating position".

SNP MP Stephen Gethins said: "The UK Government wants to use leaving the EU as cover to grab powers from the Scottish Parliament.

"This report shows there is now a real threat of a cash-grab alongside a power-grab.

"Scotland's farm payments must be protected - but the answer cannot be to rip up the Barnett formula and slash Scotland's budget for public services."

Scottish Labour leader Kezia Dugdale said the report shows keeping the integrity of the UK single market must be a "primary concern" in the years ahead.

She said: "This is an important report on the most pressing challenge facing our country today.

"Labour believes there should be a jobs-first Brexit, and we will seek a clear and binding commitment to repatriate powers in devolved areas to the Scottish Parliament.

"If there is not a clear presumption of devolution, Labour has been clear that our MPs will not support this Bill in the Commons."

Green MSP Ross Greer said: "The UK Government should be embarrassed when even the unelected House of Lords is critical of its refusal to 'respect' and 'listen' to the Scottish and Welsh parliaments and governments."

A Scottish Conservative spokesman said: "The Lords Committee is right that if we are to get the best deal, both governments need to work together - so of course the UK and Scottish Governments should engage constructively with each other throughout the Brexit process.

"But the basic fact remains that Brexit will result in more powers for the Scottish Parliament, as has been made clear by the Secretary of State for Scotland when the EU Withdrawal Bill was published.

"This report also comes to an important conclusion - that the SNP's idea for a separate deal was unworkable."