As Britain prepares to afford full state honours to the Chinese President this week, Release International says China must be held to account over religious persecution - including the disappearance and torture of Christian human rights lawyers.
Chancellor George Osborne has urged Britain and China to stick together and create a golden decade. "But for decades, life has been anything but golden for many of China's Christians," says Paul Robinson, Chief Executive of Release International, which speaks up for persecuted Christians around the world.
"Right now, one of China's most prominent Christian human rights lawyers is being held illegally in a 'black jail' on possible spying charges and denied any legal representation. Zhang Kai's crime? Offering legal advice to churches whose crosses are being torn down by the thousand as China continues to persecute its Christian minority.
"The one question for David Cameron when he meets President Xi Jinping must be: 'If you really care about human rights - when will you release Zhang Kai?' It's deeply worrying to see China rounding up its lawyers, and claiming it is doing so to uphold the rule of law."
China's Ambassador to London, Liu Xiaoming, told the Andrew Marr show at the weekend that President Xi would be coming to the UK "for cooperation, for partnership, not for a debate about human rights."
"But that debate has to take place when China is jailing and torturing its lawyers, demolishing churches, and driving Christians to worship underground," says Paul Robinson.
"And let's not forget that lawyers like Zhang Kai, who love their country, and simply want to use the law of the land to contest the legality of church demolitions are being treated like enemies of the state.
"Chinese Ambassador Liu Xiaoming insisted at the weekend: 'No-one will be put behind bars simply because they are criticising the government.' He said they were jailed because they were criminals or wanted to overthrow the government.
"And that's the problem for China: that those who question the authorities are slapped down for being enemies of the state.
"The Ambassador urged us not to miss the big picture. Well, the UK must not be so greedy for gain that we turn a blind eye to persecution. David Cameron and George Osborne have a golden opportunity to influence China for good - to call for religious freedom. And the world is watching to see they use that opportunity.
"As a growing world power, China should embrace, rather than oppose, those who stand up for justice and righteousness under the law."
Beijing-based Zhang Kai was seized after defending churches in the courts against moves to tear down their crosses. Release partner China Aid says officials in Wenzhou are charging him with spying. Release International is calling for the lawyer to be set free.
According to China Aid, Zhang has been accused of 'gathering a mob to disturb public order and stealing, collecting, purchasing and illegally providing state secrets and intelligence to overseas organisations.' China Aid fears he may be tortured into producing a false confession.
"Release is adding its voice to the growing numbers calling for the immediate release of Zhang Kai," says Paul Robinson.
There is a strong Christian presence in Zhejiang province. The communist authorities there have been demolishing churches and have pulled down some 1200 crosses. Some international observers believe the clampdown against visible signs of Christianity is intended to slow the rapid growth of the faith in China.
"'The authorities appear afraid that Chinese Christians could have divided loyalties and that human rights campaigners who mount legal challenges in the courts are undermining the authority of the Communist Party," says Robinson. "Neither is the case."
Since July, China has rounded up and detained some 230 human rights lawyers and activists. And the communist media has mounted a high-profile campaign against them, branding them as corrupt and criminal.
On August 25, Zhang Kai was arrested ahead of a meeting with the US ambassador at-large for religious freedom. Zhang was seized in Wenzhou where he was helping church leaders legally contest orders to tear down crosses from their churches.
The lawyer was charged with 'endangering national security' and 'gathering crowds to disturb the social order'. There's concern that the charge could be ratcheted up to spying, for which the maximum penalty is death. The authorities have denied access to Zhang or any legal representation.
Estimates of the numbers of Christians in China vary from 67m to 100m, potentially outstripping the membership of the Communist Party, given at 86m. According to the Council for Foreign Relations, China is on course to have the largest Christian population in the world by 2030.
Release Chief Executive Paul Robinson said: "Please pray for Zhang Kai and other courageous Christian human rights lawyers who are being arrested, detained and harassed. Pray they will know God's presence with them. Pray that they will be strengthened in their calling and know that their family around the world is standing with them."