An influential Liberal Democrat think tank will propose increasing the scope of inheritance tax (IHT) at the party conference next week, saying it had "become almost voluntary".
Currently, gifts made to family members more than seven years before the death of the donor are exempt from IHT.
However, Lord Newby, the Liberal Democrat Treasury spokesman, will say that time frame "no longer adequately reflects current life expectancies" and should be raised to at least 15 years.
The proposal is contained within Newby's report Tax and Coalition: Fairness and Responsibility?
IHT is now so easy to avoid it is seen as voluntary, Lord Newby said.
"[Current rules] no longer adequately reflect life expectancies. One of the main purposes of this rule was to allow parents to help their children in the early stages of their career.
"In many cases, parents are living into their 90s and beyond, so their children are often in late middle age when they inherit."
Newby added the time limit should first be set at 15 years before death, but could be extended further as life expectancy increases.
However, Carla Brown, senior solicitor at law firm Moore Blatch, said: "Most people who approach us for IHT planning have modest estates, are savers by nature and have been financially diligent.
"They have been net contributors to the country in taxes and want to remain financially independent until they die.
"With the prospect of ever increasing long-term care bills actively encouraging people to spend or give away money earlier because of swingeing IHT bills, this could mean the tax payer starts picking up the costs for people who would otherwise have been self-financing in their old age."
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