The Treasury has said it is supportive of Aim shares amid an ongoing review.
The Office for Tax Simplification, an independent body created to advise the government on tax matters, confirmed it is looking at the rules governing Aim shares as part of its review into inheritance tax.
The review was ordered by Chancellor Philip Hammond to investigate ways in which inheritance tax can be restructured or simplified.
It had got some advisers worried a change for Aim could be on the cards.
But a Treasury representative said: "We are fully committed to supporting Aim, which saw £5bn of shares traded on it in August 2018, and has helped thousands of UK entrepreneurs to raise the capital they need to innovate and grow."
The Treasury would not confirm its stance on IHT relief on the shares, however.
Most shares on the Alternative Investment Market (Aim) are exempt from inheritance tax as long as they are held in the estate for two years at the time of death.
This is because, despite Aim shares being listed on a stock market, they are counted as being unquoted and under the Business Property Relief (BPR) rules, shares in unquoted businesses are exempt from inheritance tax.
The rule was introduced to facilitate the transfer of family businesses to the next generation.
Shares in companies that are property businesses, or investment funds listed on Aim, however, are not eligible for the tax break.
The government has shown willingness to crack down on tax efficient investment in the past year.
The rules affecting investors in Venture Capital Trusts (VCT) and Enterprise Investment Schemes (EIS) for instance, were changed in the November 2017 Budget, when the chancellor restricted the types of companies those investors can place capital in to ensure the tax breaks were not abused by investors looking for "capital preservation".
Companies that wish to attract funding from investors in those products must now show they are "knowledge intensive" companies.
The Aim market has returned 57 per cent over the past five years, less than the return achieved by the FTSE Small Cap in the same time period.
David Scott, an adviser at Andrews Gwynne in Leeds, thinks there is scope for the rules to be tightened.
He said: "The stock market is essentially a market for second hand goods, because if you buy a share on that market, then the money goes to the person you bought them off, and they use the money for whatever they want.
"It doesn’t go to help a company grow, unless you are buying the shares at the IPO stage or via a fundraising, otherwise it is recycling capital, rather than helping businesses.
"A whole industry has sprung up based on the business property relief rules. I think putting a cap on the amount that can be invested in Aim, or other ways of tightening the rules would be appropriate."