A Sheffield MP was one of the only a handful of Labour politicians who voted against giving mandatory jail terms to people convicted of a second knife crime.
Meg Munn said she did not back the new law because it applies to people aged 16 – when the age of criminal responsibility is 17.
The Sheffield Heeley MP, a former social worker, was the only Labour MP in South Yorkshire to vote against the policy – although Liberal Democrats were also not in favour.
She said the policy did not take into consideration the country’s history of treating people as adults, in criminal responsibility terms, when they reached the age of 17.
The measures – which received widespread support in the House of Commons – mean adults will get a minimum six-month jail term on their second conviction for carrying a knife, while 16-year-olds would be given at least a four-month detaining and training order.
Mrs Munn, who said there should have been further discussions on the policy, said: “I think knife crime is extremely serious, we should never be soft on it and we need to do more to discourage young people of all ages from carrying knives and make them aware of the serious consequences.
“But I don’t think the proposal was properly thought through, drafted or that we know what the consequences of doing this would be.
“There were a number of things which worried me.”
But Barnsley East MP Michael Dugher voted for the proposals.
He said: “There is no doubt that more needs to be done to stop the devastation caused by knife crime.
“The recent knife attack in Worsbrough is a sad reminder of that.
“We need to make it very clear that carrying a knife is totally unacceptable, even more so for those who repeatedly commit knife crime.
“Tough sentencing sends out a strong signal and that’s why I supported these changes.”
The Criminal Justice and Courts Bill and its new clauses will now go to the House of Lords.
UKIP Yorkshire MEP Jane Collins added: “I am pleased that Parliament has given its backing to the move which sadly is necessary to try to prevent the tragedies caused by knife crime.”