David Cameron, along with the Treasury, the Bank of England, the International Monetary Fund and others have been attacked for exaggerating the economic risks of leaving the EU. This criticism has been widely accepted by the British media and many financial analysts. As a result, British voters are now grossly underestimating the true costs of Brexit.

Too many believe that a vote to leave the EU will have no effect on their personal financial position. This is wishful thinking. It would have at least one very clear and immediate effect that will touch every household: the value of the pound would decline precipitously. It would also have an immediate and dramatic impact on financial markets, investment, prices and jobs.

As opinion polls on the referendum result fluctuate, I want to offer a clear set of facts, based on my six decades of experience in financial markets, to help voters understand the very real consequences of a vote to leave the EU.

The Bank of England, the Institute for Fiscal Studies and the IMF have assessed the long-term economic consequences of Brexit. They suggest an income loss of £3,000 to £5,000 annually per-household once the British economy settles down to its new steady-state five years or so after Brexit. But there are some more immediate financial consequences that have hardly been mentioned in the referendum debate.

To start off, sterling is almost certain to fall steeply and quickly if leave wins the referendum. I would expect this devaluation to be bigger and also more disruptive than the 15% devaluation that occurred in September 1992, when I was fortunate enough to make a substantial profit for my hedge fund investors, at the expense of the Bank of England and the British government.

It is reasonable to assume, given the expectations implied by the market pricing at present, that after a Brexit vote the pound would fall by at least 15% and possibly more than 20%, from its present level of $1.46 to below $1.15 (which would be between 25% and 30% below its pre-referendum trading range of $1.50 to $1.60). If sterling fell to this level, then ironically one pound would be worth about one euro – a method of “joining the euro” that nobody in Britain would want.

Brexiters seem to recognize that a sharp devaluation would be almost inevitable after Brexit, but argue that this would be healthy, despite the big losses of purchasing power for British households. In 1992 the devaluation actually proved very helpful to the British economy, and subsequently I was even praised for my role in helping to bring it about.

But I don’t think the 1992 experience would be repeated. That devaluation was healthy because the government was relieved of its obligation to “defend” an overvalued pound with damagingly high interest rates after the breakdown of the exchange rate mechanism. This time, a large devaluation would be much less benign than in 1992, for at least three reasons.

First, the Bank of England would not cut interest rates after a Brexit devaluation (as it did in 1992 and also after the large devaluation of 2008) because interest rates are already at the lowest level compatible with the stability of British banks. That, incidentally, is another reason to worry about Brexit. For if a fall in house prices and loss of jobs causes a recession after Brexit, as is likely, there will be very little that monetary policy can do to stimulate the economy and counteract the consequent loss of demand.

Second, the UK now has a very large current account deficit much larger, relatively, than in 1992 or 2008. In fact Britain is more dependent than at any time in history on foreign capital. As the governor of the Bank of England Mark Carney said, Britain “depends on the kindness of strangers”. The devaluations of 1992 and 2008 encouraged greater capital inflows, especially into residential and commercial property, but also into manufacturing investments. But after Brexit, the capital flows would almost certainly move the other way, especially during the two-year period of uncertainty while Britain negotiates its terms of divorce with a region that has always been and presumably will remain its biggest trading and investment partner.

Third, a post-Brexit devaluation is unlikely to produce the improvement in manufacturing exports seen after 1992, because trading conditions would be too uncertain for British businesses to undertake new investments, hire more workers or otherwise add to export capacity.

For all these reasons I believe the devaluation this time would be more like the one in 1967, when Harold Wilson famously declared that “the pound in your pocket has not been devalued”, but the British people disagreed with him, quickly noticing that their true living standards were going down. Meanwhile financial speculators, back then called the Gnomes of Zurich, were making large profits at Britain’s expense. Today, there are speculative forces in the markets much bigger and more powerful. And they will be eager to exploit any miscalculations by the British government or British voters. Brexit would make some people very rich but most voters considerably poorer.

I want people to know what the consequences of leaving the EU would be before they cast their votes, rather than after. A vote to leave could see the week end with a Black Friday, and serious consequences for ordinary people.