Can we trust a huge investor's plea for the UK to stay in the EU?

There are a few ways of seeing the latest City claim - this one by giant money manager, BlackRock - that leaving the EU would do serious harm to our economy.

First: it's a bit scary.

BlackRock's wealth warning, in a report called "Brexit: big risk, little reward", says (errr, you guessed) that leaving the EU would bring "a lot of risk with little obvious rewards".

It predicts a reduction in growth and investment in Britain, and potentially higher unemployment and inflation.

And it argues that the £9bn odd that could perhaps be saved from no longer being obliged to contribute to Brussels' budget would be wiped out by damage - in particular - to the UK's EU-dependent financial services industry: such damage would reduce City-related tax revenues, and would worsen our already worryingly large balance of payments deficit.

If, as BlackRock expects, Brexit caused a slump in sterling, downgrades of the UK's credit rating and higher long-term borrowing costs for government, that would be an additional expense for taxpayers.

Scaremongering? Well that is what the proponents of leaving the EU will allege.

But as I have pointed out for several years, the UK's arguably too-large twin deficits - the government deficit and the current account hole in business and investment with the rest of the world - makes us perhaps riskily dependent on the kindness of foreign strangers (in the formulation of the Governor of the Bank of England).

And if BlackRock actually believes its own analysis - that Brexit reduces the ability of the UK to service its big debts - it has the power to bring about some of what it foretells: it manages a staggering $4.6 trillion of investors' funds, and if it were to reduce its investments in the UK to any significant degree, sterling and other markets would certainly feel it.

Also the authors of the report aren't thickos. One is Philipp Hildebrand, former chairman of the board of governors of the austere and conservative Swiss National Bank.

Another is Rupert Harrison.

He of course is luscious fresh meat for eurosceptic conspiracy theorists - because till last summer he was the highly influential super brainy special adviser to George Osborne.

So Harrison can be very easily characterised as a fully paid up member of David Cameron's and Osborne's Project Fear - or the putative campaign to worry us that leaving the EU is a shortcut to national penury.

Which brings us perhaps to the central paradox of the BlackRock report.

It can be interpreted as special pleading by a financial services industry that loves the EU single market and does a ton of business out of London for businesses and citizens all over the EU - and also as the last word on why Brexit would muller an important British industry.

Many City firms, for all their wariness of Brussels rules and laws that seem often expressly designed to crush their profitability, love the passport that British EU membership confers on them - a passport to trade freely with the EU's business and its 500m people.

So BlackRock and they say leaving the EU would damage them.

That makes them a biased commentator on whether we should Remain or Leave, and simultaneously an expert witnesses on the harm to our prosperity of EU membership

The point is that financial services - whether we like it or not - represent something over a tenth of our entire economy.

You presumably don't need telling that there is a compelling argument that we are too dependent as a nation on the City.

But most would say that it would be a bit drastic to quit the EU expressly to force a bunch of banks, insurers and fund managers to emigrate or wind down - especially if that was a re-balancing through shrinking the economy rather than growing it.