Shadow chancellor Jeremy Hunt has warned hiking up tax could "destroy growth".
Speaking at the Conservative Party Conference in Birmingham, Hunt said changes to capital gains tax could deter needed investment in the economy.
Hunt also said he was "pretty dubious" of Rachel Reeves' £22bn black hole claim.
When asked whether he agrees with Reeves that things will have to get worse before they get better, he said: "They will get worse if Labour makes catastrophic mistakes in the Budget and hikes up tax in a way that destroys growth.
"I think one of the biggest lies we've had since Labour came to office is this nonsense about having the worst economic inheritance since the Second World War."
He said countries growing most quickly in North America and Asia were those with lower tax burdens than the UK.
He pointed to welfare reform as a way to bring down this burden.
Hunt added: "My worry about the last 12 weeks is that Labour have got themselves so badly burnt with the mess they've got into over winter fuel allowance that welfare reform will now become untouchable for them.
"If they worried about a battle with the Labor Party on benefits paid to pensioners, including wealthier pensioners, you can imagine how challenging it will be to tackle the bill for working age disabled people, which is due to increase by £25bn a year over the next five years.
"The argument we Conservatives make is that if you do have mental health issues, one of the worst possible things is to be signed off having to look for work, because that means your isolation is likely to increase and your mental health problems are likely to get worse.
"So that is a very, very crucial area, both for public finances and also if you want to reduce the dependence on immigration in the economy, that really needs to be tackled."
He said the Conservative Party would be pushing for these "difficult decisions" to be made.
tara.o'connor@ft.com
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