The UK last night demanded “urgent verifiable evidence” from China that missing tennis star Peng Shuai is safe and well.
Chinese state media ran a video on Saturday that appeared to show the 35-year-old in a restaurant, after she made allegations of sexual assault against a former vice-premier in China.
But the Foreign Office said the UK Government remained "extremely concerned" and was following the case closely.
A spokesperson said: "The Chinese authorities should urgently provide verifiable evidence of her safety and whereabouts.
"Everyone should be allowed to speak out without fear of repercussions. All reports of sexual assault, anywhere in the world, should be investigated."
It comes amid reports the UK is looking at the idea of staging a diplomatic boycott against the 2022 Winter Olympics in Beijing.
Peng, a former world number one in doubles and Wimbledon champion, made the allegations on the social media site Weibo
According to the Sunday Times, Britain is in discussion with its Five Eyes security allies about the idea.
US President Joe Biden may announce he and US officials will stay away from the Games over China’s human rights record.
Ministers are in "active discussions" about whether to follow suit with representatives of Australia, Canada and New Zealand, the newspaper reported.
Peng, a former world number one in doubles and Wimbledon champion, made the allegations on the social media site Weibo.
Since, she has not been heard from directly for more than fortnight, with concerns for her safety amplified as far as the United Nations and the White House.
Former Conservative Party leader Sir Iain Duncan Smith told BBC Radio 4's PM programme that the UK should "stand up" and make it clear to China that human rights abuses we unacceptable.”
He added: "What we should be doing absolutely is coming out clearly now and saying we are, as a government, no officials and no ministers, including embassy staff (will) go to the Winter Olympics.
General view of Beijing Winter Olympic Village for next year's competition
China News Service via Getty Ima)
"We should come out with that straight away, not wait for somebody else to do it. That would send a very strong signal to them."
Sir Iain said the UK should then consider "all other sporting engagements" with what he labelled a "brutal regime that is arresting, incarcerating, peaceful demonstrators".
He said: "We have to stand up and say we're not going to put up with this anymore."
White House press secretary Jen Psaki told reporters on Friday: "We are deeply concerned by reports that Peng Shuai appears to be missing after accusing a former PRC (People's Republic of China) senior official of sexual assaults.
"We join in the calls for PRC authorities to provide independent and verifiable proof of her whereabouts and that she is safe."
Liz Throssell, the spokesman for the UN Human Rights Office, echoed those concerns and called for an investigation "with full transparency" into Peng's allegations.
Ms Throssell told reporters: "(Peng) hasn't been heard from publicly since she alleged on social media that she was sexually assaulted.
"What we would say is that it would be important to have proof of her whereabouts and wellbeing, and we would urge that there be an investigation with full transparency into her allegations of sexual assault."
The Women's Tennis Association has threatened to pull out of the lucrative Chinese part of its tour next season unless it is proved that Peng is safe.
WTA chairman Steve Simon told the BBC: "We cannot stand by compromises. This is a right and wrong issue."
The reason a diplomatic boycott would work, Sir Iain said, is because China is "very sensitive about how the outside world… behaves towards them.
"They're sensitive when governments boycott things because these sort of sporting engagements are a big deal for China. They make a big fuss about it. When they had the Olympics they made a big fuss about it. It makes them look like they're acceptable to the world.
"And our problem is we play that game and we must no longer play that game. We must simply say we're not prepared to have any diplomatic relationship with you over these games."
But he added: "We are dealing with a country that has no value at all for human life if you criticise it, the concept of the rule of law and human rights is gone, and we must now stand up."