Families have just a few weeks to reorganise their affairs before another of Gordon Brown’s trust crackdowns comes into force next month.

In the 2006 budget, Brown, then chancellor, hit two popular types of trust with a 20% tax charge. The clampdown caused an outcry because it was retrospective, hitting existing as well as new trusts, so the government gave families until this year to rearrange their schemes and avoid the penalty.

The two types of trust affected were so-called accumulation and maintenance trusts — popular with grandparents — and life-interest or interest-in-possession trusts.

The latter are generally used when you want to leave your capital and income to different people. You might make your surviving spouse entitled to the income, for example, while your children get the capital. That way your capital is protected if your spouse remarries.

From October, existing trusts will be subject to a 20% tax charge above the nil-rate band of £312,000 if the terms are changed, perhaps because you want to nominate a different beneficiary. Families are therefore being urged to make any alterations ahead of the deadline.

John Wray of law firm Wedlake Bell said: “Getting all the right ducks in a row to transfer a trust interest is likely to be a time-consuming process. With only a matter of weeks to go before the rules change, anyone considering passing on their life interest trust needs to act quickly.

“Failure to make the most of this window of opportunity before it closes could result in a hefty and unnecessary inheritance-tax bill. If similar action is taken after October 5, it could cost trusts of significant value many thousands of pounds and substantially deplete the assets of the trust.”

Altering the terms of a trust deed can be complicated as you will need to consult the trustees as well as both the previous and new beneficiaries.

There are plenty of ways round the crackdown. If gifts into trust, or transfers, are below £312,000 there is no immediate tax charge. Once you have used the nil-rate band, you cannot do it for seven years, so advisers recommend you start planning to give assets away as soon as possible.

Only gifts into trust out of capital are subject to the 20% tax. Gifts out of income are automatically exempt from IHT if they are regular and do not reduce your standard of living.

Discounted gift trusts can also escape the clampdown. They allow you to give away assets while retaining the right to draw a regular income. The fact that you retain a right to income reduces the value of your initial gift into the trust — in some cases below the £312,000 threshold.