A FRESH warning about the potential economic cost of Brexit has been issued by the head of the World Trade Organisation, who said import tariffs would cost the UK billions. WTO director general Roberto Azevedo said the UK would have to negotiate membership of the organisation – where it is currently represented by the EU – and trade deals with countries around the world.

The intervention came as the rival camps in the referendum debate clashed over a stark warning about the potential impact of Brexit from the Institute for Fiscal Studies (IFS). David Cameron said the think-tank is the “independent gold standard” but Leave campaigners claimed it was influenced by funding from Brussels.

The WTO estimates the cost of additional tariffs on goods imports to British consumers after Brexit would amount to £9 billion, while British merchandise exports would be subject to a further £5.5 billion in tariffs. Azevedo told the Financial Times: “The consumer in the UK will have to pay those duties. The UK is not in a position to decide

‘I’m not charging duties here’.

That is impossible. That is illegal.”

Setting out the scale of the challenge facing the UK if it voted to Leave on June 23, Azevedo said: “Pretty much all of the UK’s trade would somehow have to be negotiated.”

Brexit campaigners have suggested that the UK would pull out of the single market in the event of a Leave win, and Britain could face being cut out of existing trade deals negotiated between the EU and other countries.

Azevedo said: “It is a very important decision for the British people. It is a sovereign decision and they will decide what they want to decide. But it is very important, particularly with regard to trade, which is something very important for the British economy, that people have the facts and that they don’t underestimate the challenges.”

The intervention by the WTO chief follows warnings from other major international economic bodies. The IFS think-tank forecast Brexit could lead to two more years of austerity.

Its report said a vote to Leave could see public finances take a £20bn to £40bn hit in 2019/20, if gross domestic product is 2.1 per cent to 3.5 per cent lower over the period, as predicted by the National Institute of Economic and Social Research. It also rejected a key claim from the Leave camp that EU membership involves sending £350m a week to Brussels which could be used in the UK. Cameron said as he arrived in Japan for the G7 summit: “What they are saying about the £350m claim and what they are saying about the effect upon our economy of Brexit, that is very, very powerful and backs up what the Treasury and others have been saying,”

Vote Leave dismissed the IFS as a “paid-up propaganda arm of the European Commission” which could suffer a financial hit from Brexit because it benefits from EU funds.

The campaign said: “The IFS is not a neutral organisation. It would face an £800,000 deficit if we vote Leave.”

IFS director Paul Johnson said it received European funding for some of its “academic work”, as do other institutions outside the EU.

He said: “For the last 30 years, the IFS has built its reputation on the independence and integrity of our work. There is no sum of money from anywhere in the world that would influence what we said because, if it did, then the point of the IFS and the reasons we are listened to after budgets and so on would simply be lost.”