If Britain crashes out of the European single market for goods and services in the wake of the Brexit vote the country could be permanently poorer by 4 per cent of GDP, according to estimates from the Institute for Fiscal Studies.
In today’s money that loss equates to roughly £75bn, or £2,900 for each of the country’s 26 million households.
The nature of Britain’s future trade relations with the rest of the EU remain deeply uncertain.
Before the referendum some leading Leave campaign figures suggested Britain could retain membership of the single market while being outside the EU in the manner of Norway.
But other senior European politicians and officials have made it clear they will not accept this arrangement unless Britain continues to make a sizeable contribution to the EU budget and also continues to allow the free movement of European workers into Britain (as Norway does).
The signals from Theresa May’s government in recent weeks suggest the “Norway” option is politically impossible and that ministers are preparing to leave the single market and attempt to generate some kind of free trade agreement with the rest of the bloc.
If a free trade agreement is not agreed the UK could default to trading goods with the rest of the EU on minimal World Trade Organisation (WTO) rules.
But the IFS estimates that retaining single market membership could be potentially worth 4 per cent of GDP permanently to the UK economy relative to WTO terms by 2030 - equal to two full years of trend growth.
They based this figure on long-term estimates from other forecasting organisations such as the London School of Economic's Centre for Economic Performance and the National Institute for Economic and Social Research (NIESR) of the long-term negative impact of Brexit on UK growth.
The research organisation also stresses that the cost of losing membership of the single market would far outweigh the economic benefits of the UK no longer paying in a net £9bn a year into the European Union budget.
“The macroeconomic impacts of membership and access are much larger than the importance of direct budgetary issues, even relative to the UK’s full EU contribution” it said.
A report from the IFS published before the referendum said lower growth and extra borrowing resulting from the shock of a Leave vote could knock a £20bn to £40bn hole in the Government’s finances by 2020, potentially extending government austerity policies into the next decade.
This modelling exercise was also based on the post-Brexit vote (short-term) economic forecasts from NIESR.
“From an economic point of view we still face some very big choices in terms of our future relationship with the EU” said Ian Mitchell, a research associate of the IFS in the new report.
“There is all the difference in the world between ‘access to’ and ‘membership of’ the single market” he added.
The IFS said free trade agreements are designed to tackle trade tariffs, whereas the EU single market also aims to dismantle all non-tariff barriers to trade such as licensing, and was, thus, much more valuable to the UK’s exporters.
Dismantling non-tariff barriers are especially important to services firms, which make up 80 per cent of the UK's output. In 2015 39 per cent of the UK's services exports - worth £89bn - went to the rest of the EU.
Almost all economic forecasters expect the UK leaving the EU to have a long-term negative impact on the economy, with their views of the size of the loss largely varying depending on what trade arrangement substitutes for full single market membership.
One economic model – from Patrick Minford of Cardiff University – forecasts a boost to UK GDP from leaving the single market and instantly abolishing all import tariffs – but other economists have strongly criticised his model’s underlying assumptions and Minford himself admits his recommended policy would wipe out much of the UK’s manufacturing industry.
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