Chancellor Philip Hammond says National Insurance is being increased for self-employed workers because of "new challenges" the government faces.
Mr Hammond is facing a backlash against the Budget announcement, which breaks a 2015 manifesto pledge on tax rises.
He insisted ministers had honoured a "broad commitment" not to raise taxes.
Labour urged the government to "think again" on the change, which will mean 1.6 million people paying £240 on average more every year.
Shadow chancellor John McDonnell said his party could join forces with rebel Conservatives who say the increase does little to encourage enterprise and risk-taking.
Wales minister Guto Bebb told BBC Radio Cymru he believed the party should apologise to voters for breaking their manifesto pledge. He said, speaking in Welsh, "I will apologise to every voter in Wales that read the Conservative manifesto in the 2015 election."
Fellow Conservative, Stephen McPartland MP, described the measure as "unacceptable", saying it sent out the wrong message to ordinary working families. He appealed to the chancellor to do a "U-turn" quickly before a manifesto promise was broken.
But Mr Hammond defended the move in an interview with BBC Radio 4's Today programme.
Asked about the Tories' pre-election pledges not to increase National Insurance, he said: "There was a broad commitment to lock taxes so there would be no tax increases. That's what we have done."