Keir Starmer today said he has "concerns" over Scotland's gender recognition reforms, because he believes 16 is too young for someone to change their legal gender.
But he refused to say if he would overrule the new law if he was Prime Minister - as Rishi Sunak could decide to do this week.
The SNP government in Scotland passed legislation last month which will allow trans people to obtain a gender recognition certificate without the need for a medical diagnosis.
The Gender Recognition Reform (Scotland) Bill also lowers the minimum age for applicants from 18 to 16.
And it drops the time required for an applicant to live in their acquired gender from two years to three months - six for those aged 16 and 17 - but with a three-month reflection period.
Rishi Sunak has voiced concerns about the law. In a constitutional clash, he could use a Section 35 order this week to block it from gaining Royal Assent.
Mr Starmer said he wants to 'modernise' the law for trans people to change their legal gender and remove 'indignities’.
He would not be pinned down on specifics, adding the issue has become a 'toxic political football’.
But he did confirm “I don’t think you are” old enough at 16 to change your legal gender - a key plank of the Scottish government’s new law.
Mr Starmer the BBC's Sunday With Laura Kuenssberg: "I have concerns about the provision in Scotland, in particular the age reduction to 16 and in particular the rejection of our amendment in relation to the Equalities Act.
"But across the whole of the area, I think we should modernise the law.
"I think we need a respectful debate that recognises the different arguments that are being made.
“At the moment, this is being treated as a political football from start to finish and I don't think that actually advances the cause of anyone."
Pressed on whether he thinks someone of 16 is old enough to decide to change their gender, he replied: "No, I don't think you are."
The UK Government is currently considering legal advice about whether to use its powers to block the Bill from becoming law.
Transport Secretary Mark Harper said ministers are awaiting "detailed analysis" of how Scotland's gender law will affect UK legislation, such as the Equalities Act.
He told the BBC : "We are not proposing to make those changes for England, but what we have to do is make a decision about whether that legislation impacts on legislation elsewhere in the UK.
"One of those pieces of legislation is the Equalities Act.
"That is why we need a detailed analysis of that, and that is the information the Government needs before it can take a decision."
Sir Keir, asked whether he would block the Scottish legislation if he was prime minister, said he wants to wait to see what legal advice the UK Government has received.
Scotland Secretary Alister Jack has, in consultation with Downing Street, until this week to decide whether to trigger Section 35 of the Scotland Act for the first time.
Stephen Flynn, the SNP's Westminster leader, said it would be an "outrage" if London did choose to intervene, saying it would amount to the UK Parliament "overstepping massively".
Speaking to the BBC's Sunday Show, Mr Flynn said: "We elect members to the Scottish Parliament to create legislation to put in place the laws to enhance the lives of people in Scotland.
"The GRR (Gender Recognition Reform) will do just that.
"What we have is a right-wing UK Conservative Government which is seeking to roll back on the democratic powers of the Scottish Parliament - that's an outrage."
Conservative MP Caroline Nokes, chairwoman of the Women and Equalities Committee, told the BBC: "I absolutely believe we have to make the process simpler and kinder.
"But I really fear the use of this as some sort of woke culture war."
