While the main features of share schemes in private companies will operate just as they would in listed firms, a couple of key differences continue to separate the two. First, it will be hard for a private firm to place a value on the stock they choose to hand over to their workers because of the absence of a quoted price.

Secondly, since shares in a private company can be difficult to buy and sell, an employee may have problems in realising the true value of their investment.

Ms Doggett says the "challenges" created by these two issues have been given an increased focus by the publication of this year's Finance Bill, the legislation that gives effect to the Budget measures.

The Bill, which is due to become law early next month, abolishes an existing facility whereby income tax due on the exercise of share options can be deferred for a relatively lengthy period.

This will, says Ms Doggett, create particular problems for employees exercising share options in private companies.

Previously, such workers could generally defer tax due on the exercise of their options until they sold the shares, or for seven years, whichever came earlier.

This meant that employees were afforded a period of time in which to build up the funds necessary to pay their tax bill.

Under the new system, however, the tax will need to be paid within 30 days, leaving employees owing tax on an investment they can not readily trade on an open market.

Ms Doggett says the practical effect of this measure is to place pressure on employers to create a facility whereby the shares can be sold.

Options here include a company buyback or a repurchase of the shares by a parent company, mechanisms that can be established relatively simply under professional guidance.

Ms Doggett says the significance of valuation is also heightened in the new legislation.

"Without this information, the employee will not be in a position to calculate his gain and thus the income tax payable," Ms Doggett says, adding that penalty charges can arise where the full amount of income tax is not payable on time.