Consumers face "inevitable" price rises on food and drink if the Government opts for "hard Brexit", former deputy prime minister and Lib Dem leader Nick Clegg has warned.

Grocery bills will have to bear the knock-on costs of "whopping" tariffs on imported foods imposed if the UK leaves the European single market, with a 59% levy on beef, 38% on chocolate, 40% on New Zealand lamb and 14% on Chilean wine, said the former Liberal Democrat leader.

And the collapse in the value of sterling will add to the pain - with the pound's fall from pre-referendum levels already well beyond the 12% point which would add £123 to the average family's shopping bill, according to Treasury estimates.

Mr Clegg, who has been appointed Lib Dem EU spokesman by party leader Tim Farron, said that "hard Brexit" - in which the UK leaves the single market and customs union and trades under World Trade Organisation rules - would create "turmoil" in Britain's food and drink industry.

For the sector - which employs 850,000 and is heavily dependent on EU markets and migrant labour - hard Brexit is "not a sunny upland, it's a cliff edge", he warned in a speech in London.

Food and drink exporters will face the return of customs checks at EU borders, requiring import licences, health certificates and vets' inspections to show their products comply with EU rules, said Mr Clegg.

Farmers will see average tariffs of 22.3% slapped on their produce for export, and will "most likely" lose some of the £3 billion in subsidies they currently get from Brussels.

Companies wanting to trade with continental Europe will have to continue to comply with EU rules on issues like labelling or safety standards, but will no longer be able to influence them.

And manufacturers of processed food will have to deal with big price hikes in raw ingredients, both from the UK and abroad.

The row which saw products like Marmite and PG Tips briefly withdrawn from supermarket shelves was "a foretaste of what is to come in the next 12 months", said Mr Clegg in a speech in London.

The food and drink industries are a "bellwether for a successful Brexit", he said, adding: "If the Government can't get it right for them, then they won't be able to get it right for other sectors.

"By far the least worst of the options on offer" following the June 23 vote to leave the EU would be for Britain to join Norway, Iceland and Liechtenstein, who are within the single market but not full members of the 28-nation bloc, said Mr Clegg. These countries are required to observe freedom of movement rules and pay into EU budgets in return for tariff-free trading.

He accused Theresa May's Government of taking an "autocratic and unconstitutional" approach by denying MPs a vote on the UK's stance in the upcoming withdrawal negotiations under Article 50 of the EU treaties.

"At the moment we are being led into a crisis of the Government's own making," said Mr Clegg.

"But there is an alternative. In fact, it is the only alternative to the scenario I have outlined today. That is for the Government to opt to keep us in the single market by pushing for a Norway-style deal, either as an end point or as a transitional status pending the ironing out of all our new trading relationships.

"It wouldn't be popular with the Eurosceptics, and it certainly wouldn't be easy to negotiate with our European partners. But this Government has a duty to try.

"So I call on the Government to have the courage to stand up for the single market. If they don't, then Parliament may just force them to do so."