Labour person Sir Keir Starmer believes 16-year-olds are excessively young to alteration their legally recognised gender.
The UK Labour person voiced "concerns" astir the Scottish government's reforms to the process, citing a imaginable interaction connected UK-wide equalities law.
However, helium stopped abbreviated of backing a situation to the Holyrood legislation, thing UK ministers are considering.
The SNP's Westminster person Stephen Flynn said immoderate determination to artifact the measure passed by MSPs would beryllium "an outrage".
The Scottish authorities has said it volition "vigorously" contention immoderate challenge.
The Gender Recognition Reform (Scotland) Bill, passed by MSPs, removes the request for radical to get a aesculapian diagnosis of sex dysphoria earlier starting the alteration process.
It besides drops the property bounds to 16, and cuts the magnitude of clip the process takes from 2 years to a substance of months.
Scottish Labour supported the reforms, and astir each of its MSPs voted for the finalised bill.
But Sir Keir told the BBC's Sunday with Laura Kuenssberg: "I person concerns astir the proviso successful Scotland, successful peculiar the property simplification to 16 and, successful particular, the rejection of our amendment successful narration to the Equalities Act."
Pressed connected whether helium thinks idiosyncratic of 16 is aged capable to determine to alteration their gender, helium replied: "No, I don't deliberation you are."
Scottish Labour had tabled an unsuccessful amendment that sought to clarify the exertion of UK-wide equality laws successful areas specified arsenic harmless spaces.
Sir Keir besides told the programme a respectful statement was needed connected the contented and said helium believed it was presently being treated arsenic a "political football".
Asked to clarify his position, the Labour person replied: "Modernise the authorities to instrumentality retired the indignities."
The UK authorities is considering challenging the legislation, with Prime Minister Rishi Sunak saying it is "entirely reasonable" for ministers to analyse the imaginable ineligible interaction of the measure connected the remainder of the UK.
Sir Keir would not beryllium drawn connected whether helium would backmost a challenge, saying helium wanted to hold and spot what UK ministers decided to do.
The UK authorities is presently considering ineligible proposal astir whether to usage its powers to artifact the measure from becoming law.
Prime Minister Rishi Sunak spoke during a sojourn to Scotland past week astir the concerns helium has implicit the changes it would enact.
Downing Street officials said connected Saturday that the afloat ineligible proposal to ministers had not yet been reviewed and nary decisions had been made.
UK Transport Secretary Mark Harper was besides asked astir the sex self-identification laws being changed successful Scotland.
He said ministers were awaiting "detailed analysis" of however Scotland's sex instrumentality would impact UK legislation.
Mr Harper told the BBC: "We are not proposing to marque those changes for England, but what we person to bash is marque a determination astir whether that authorities impacts connected authorities elsewhere successful the UK.
"One of those pieces of authorities is the Equalities Act.
"That is wherefore we request a elaborate investigation of that, and that is the accusation the authorities needs earlier it tin instrumentality a decision."
'That's an outrage'
Mr Harper added that transgender radical had received maltreatment and their rights should beryllium respected but women besides had concerns astir risks to their safety.
The curate besides described disapproval of the writer JK Rowling, who has condemned the authorities successful Scotland, arsenic unfair.
Meanwhile, SNP Westminster person Stephen Flynn defended the reforms.
He told BBC Scotland's The Sunday Show: "What we person is simply a right-wing UK Conservative authorities which is seeking to enactment backmost connected the antiauthoritarian powers of the Scottish Parliament. That's an outrage.
"And the radical who should beryllium astir outraged by that are the Conservative and Labour politicians who voted successful favour of the GRR (Gender Recognition Reform) who indispensable recognise the information this is the UK parliament overstepping massively."
The Scottish Greens criticised the Labour person and said his comments were a "shameful intervention".
Equalities spokeswoman Maggie Chapman said: "Starmer is ignoring the views of the immense bulk of the Scottish Parliament, including the Labour MSPs who rightly backed the bill.
"A batch of radical successful Scotland volition ne'er forgive him if helium lines up with the Tories to artifact what is simply a tiny but important measurement for equality."