The Labour leader also said he was concerned about the impact on the UK-wide Equality Act. The comments put him at odds with the party in Scotland, which backed the legislation during a vote in the Scottish Parliament before Christmas and come as the UK government considers whether to use powers to block the law gaining Royal Assent.
Scottish Secretary Alister Jack has, in consultation with Downing Street, until Wednesday to decide whether to trigger Section 35 of the Scotland Act for the first time.
MSPs passed the Gender Recognition Reform (Scotland) Bill by 86 votes to 39, approving reforms which would allow trans people to obtain a gender recognition certificate (GRC) without the need for a medical diagnosis.
The bill will also allow 16 and 17-year-olds to apply for a GRC for the first time, and would reduce the amount of time a person has to live in their acquired gender before they can be granted the document.
However, the move has sparked controversy, with concerns from some politicians, women’s rights groups and others that the changes could impact on safe spaces for females.
Sir Keir said the Labour Party believes the gender recognition process needs to be modernised, but added that for “99.9 per cent of women the issue is biological” and it is important to preserve things like safe spaces.
Speaking yesterday, he said: “There are a small number of people who don’t identify with the gender that they’re born into. There is a process that they can go through at the moment.
"There are indignities in that process that I think could be improved through modernising the legislation, but what I don’t want to get drawn into is the usual, toxic political football that this always seems to become.”
Asked if he would mirror Scotland’s legislation, he said: “We want to modernise the system. We’re looking at what the options are. There are all sorts of different definitions in relation to self-certification.
"So far as the Scotland provision is concerned, I do have concerns about one, the age of transition – reduced now to 16, and we put amendments forward in relation to that – and also the primacy of the Equality Act, which is very important when it comes to things like safe spaces.”
Asked if Scottish Labour were wrong to support the legislation, Sir Keir said: “Well, that was a matter for Scottish Labour. I’m telling you what the position is in relation to the whole Labour Party. Our position is that we want to modernise the legislation and to make sure that some of the indignities that are there in the process are taken out of the process.”
Asked if people are old enough at 16 to change their legal gender, he said: “No, I don’t think you are.”
The UK Government is currently considering legal advice on whether to use its powers to block the bill from becoming law. During a visit to Scotland last week, Prime Minister Rishi Sunak said he was concerned about its potential impact.
He told the BBC: "Obviously this is a very sensitive area and I know there were very robust debates and exchanges on it as the bill was passing in Scotland.
"What I'm concerned about is the impact of the bill across the United Kingdom. As is entirely standard, the UK Government would take advice on that. There may be impacts across the UK that we need to be aware of and understand the impact of them.
"That is what we are doing, and once the Government has received final advice it will set out next steps."
Asked if he would block the legislation if he was prime minister, Sir Keir said he would wait and see what legal advice the UK Government has received.
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: “It is a devolved matter for the Scottish Parliament to determine, and the Scottish Parliament has determined its position. 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) is going to 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.”
He added: “The reality here is the Scottish Parliament has voted for something. That should happen under the devolution settlement.”
Scottish Green MSP Maggie Chapman called Sir Keir’s comments “shameful” and accused him of showing “a total disregard for the rights of trans people in Scotland and beyond”.
She said: "He is repeating toxic Tory talking points and legitimising the views of a hostile and anti-trans government that is threatening to overrule the Scottish Parliament. This is also an issue of democracy. Starmer is ignoring the views of the vast majority of the Scottish Parliament, including the Labour MSPs who rightly backed the Bill.
"The message that he is sending is that trans rights are not safe under Labour and nor is devolution. A lot of people in Scotland will never forgive him if he lines up with the Tories to block what is a small but important step for equality.”
The Scottish Government was contacted for comment.