The warning comes at a politically sensitive moment for the Government, with the Budget approaching and pressure mounting on ministers to find additional revenue to deal with Britain’s difficult fiscal position.
For Labour, the attraction of increasing Capital Gains Tax is obvious. Raising the amount paid by people who make profits from selling assets could potentially generate additional revenue without directly increasing income tax rates for millions of employees.
But economists and business groups have repeatedly warned that taxes on investment behave differently from taxes on wages.
Someone earning a salary generally has little ability to decide when their income is received. Investors, entrepreneurs and property owners, however, can often choose when to sell an asset.
That creates an important problem for the Treasury.
If the tax rate becomes significantly higher, an investor may decide to delay a sale, restructure an investment or hold an asset for longer. An entrepreneur considering selling a company may also decide that the financial reward is no longer sufficient to justify the transaction.
In other words, the Government cannot simply assume that a higher rate automatically produces proportionally higher tax receipts.
This is the central argument behind the Institute of Economic Affairs warning that a proposed increase could ultimately cost the Exchequer around £6.2billion over the remainder of the Parliament.
The argument is based on a long-standing principle in economics: taxpayers respond to incentives.
When the tax on a particular activity increases, people may do less of that activity.
In the case of Capital Gains Tax, the consequences could be particularly significant because the tax is generally triggered when an asset is sold and a gain is realised.
An investor who believes that selling today will result in a much larger tax bill could simply wait.
That does not necessarily mean the asset becomes more valuable to the economy. Instead, capital can become locked up in existing investments rather than being redirected towards new businesses and projects.
This is where the political argument becomes broader than the Budget itself.
Britain is already trying to encourage more investment, entrepreneurship and business growth. Ministers want companies to expand, create jobs and improve productivity.
Critics of a CGT increase argue that imposing a substantially higher tax on successful investment sends the opposite message.
They fear that international investors could become more cautious about committing money to Britain, while wealthy individuals and entrepreneurs may look at alternative jurisdictions where the tax treatment is more favourable.
The Government could therefore find itself caught in a difficult situation.
It needs additional money to balance the books, but it also needs economic growth to increase tax revenues over the longer term.
If a tax increase weakens investment and entrepreneurship, the short-term revenue gain could potentially be offset by slower economic activity.
That is why the £6.2billion warning has attracted attention.
The figure is not simply an argument about how much taxpayers would pay. It is an argument about how much money the Treasury might fail to collect because taxpayers change their behaviour.
Supporters of a higher CGT rate, however, are likely to take a very different view.
They can argue that people who make substantial gains from investments should contribute more towards public services, particularly when the Government is under pressure to maintain spending on the NHS, social care, infrastructure and other priorities.
From that perspective, asking investors to pay more when they make large gains could be presented as a question of fairness.
Labour also faces pressure from sections of its own support base to ensure that the burden of repairing the public finances does not fall disproportionately on ordinary workers.
That creates a political incentive to focus on taxes affecting wealth and investment rather than simply increasing taxes on employment.
The difficulty is determining where the line should be drawn.
If the Government sets the rate too low, critics can accuse ministers of failing to raise enough money from those with the greatest ability to pay.
If it sets the rate too high, critics can argue that it discourages the very investment Britain needs.
The experience of previous tax changes also means that the Treasury will have to consider behavioural responses carefully.
A tax system does not operate in isolation. Investors can change the timing of transactions, entrepreneurs can alter corporate structures and wealthy individuals can change where they live or where they invest.
Even when the Government has no intention of driving people away, a large increase in the tax burden can influence decisions at the margin.
The Adam Smith Institute has gone further, arguing that Britain should consider reducing or even abolishing Capital Gains Tax rather than increasing it.
That is a much more radical proposition, but its supporters claim that removing the tax could encourage investment and economic activity sufficiently to generate stronger growth.
The Government is unlikely to embrace such an approach given the current pressure on the public finances.
Britain’s debt burden remains high, while the cost of servicing government borrowing places further pressure on spending plans.
That leaves the Chancellor facing a difficult choice ahead of the Budget.
Every potential tax increase carries political consequences.
A rise in Capital Gains Tax could anger investors, business owners and entrepreneurs. But abandoning the idea could leave ministers searching for billions elsewhere.
That could mean higher taxes on consumption, employment or property, spending cuts, or increased borrowing.
None of those options is politically easy.
The issue also matters to ordinary workers even if they never personally pay Capital Gains Tax.
Businesses rely on investment to expand. New companies require capital to hire employees, purchase equipment and develop products.
If investors become less willing to provide that capital, start-ups could find it harder to grow.
A successful entrepreneur eventually selling a company can also release capital that is reinvested elsewhere. Critics of higher CGT argue that making those transactions less attractive could reduce the flow of money through the economy.
That is why Mr Boboc warned that ordinary workers could ultimately pay the price of a tax aimed primarily at investors.
The Government, however, will argue that public finances cannot be ignored.
Higher borrowing costs mean ministers must demonstrate that they have a credible plan for balancing revenue and expenditure. Investors in government debt also need confidence that Britain can control its finances.
The Chancellor therefore has to weigh two competing risks.
One is the risk of failing to raise enough money to meet fiscal commitments.
The other is the risk of raising taxes so aggressively that economic activity weakens and future revenues suffer.
The £6.2billion estimate has intensified that debate because it suggests the second risk may be larger than some policymakers expect.
Ultimately, the decision will come down to what John Healey believes will deliver the strongest combination of revenue, investment and economic growth.
The Chancellor could still decide to leave Capital Gains Tax unchanged, introduce a more modest increase, or pursue broader reforms designed to reduce opportunities for tax avoidance while limiting the impact on productive investment.
For taxpayers and businesses, however, the uncertainty itself is becoming an issue.
Investors make decisions based not only on today’s tax rates but also on expectations about what governments might do tomorrow.
A Budget perceived as hostile to investment could influence decisions long before any new tax rate actually takes effect.
That is why the Chancellor’s next move will be closely watched by the financial markets, business leaders and households across Britain.
The Government needs more revenue. But critics are warning that simply increasing the tax rate may not provide the answer.
If investors respond by delaying sales, reducing investment or moving capital elsewhere, the Treasury could discover that the supposedly easy tax raid is anything but.
And with the public finances already under intense pressure, the Chancellor can ill afford to make a decision that leaves Britain with both higher taxes and weaker growth.