Rebel Tories were poised to back a plan for British courts to rule if a deal should be blocked – but an arcane procedure denied the vote in favour of handing the decision to parliamentary committees instead.
A Uighur exile said she was “sickened” by the “shameful” tactics, with the revolt aimed primarily at China amid evidence it is committing genocide against the minority Muslim group.
And Tory rebels also expressed outrage, Nusrat Ghani saying she was “appalled at the Parliamentary games played over an issue as grave as genocide”.
Iain Duncan Smith, the former party leader, said: “Instead of a beacon of light and hope, today what we have done is gone into the dark corridors of procedural purdah and we need to emerge.”
Earlier, Benedict Rogers, the chief executive of Hong Kong Watch, described the tactics as outrageous, adding: “I have never seen a government so afraid to do the right thing.”
The Alton amendment to the Trade Bill, passed in the House of Lords last week with a huge majority of 171, would have given judges the role of deciding if potential trade partners are guilty of genocide.
But ministers coupled that vote with a separate Labour-sponsored amendment to impose further checks before trade deals are signed – diluting Tory support.
Even so, a government-backed “compromise” to give Commons and Lords committees powers to investigate whether a genocide is occurring passed with a majority of just 15, after a big Conservative revolt.
The close result may persuade peers to continue their resistance when the legislation returns to the upper chamber.
Ms Ghani had protested that the committees could already prepare reports on genocide, making the offer “meaningless since genocide can only be determined by a judicial process”.
“Let the record show that, on this day ,men and women in this House were ready to vote on the genocide amendment, to lead the world in standing up to tyrannical regimes who commit genocide, to honour our vow of never again, to ensure we are never complicit in genocidal trade and to put Britain on the right side of history – and today we were denied that vote,” she said.
The genocide allegations against China cannot be referred to the international courts because Beijing has a veto, prompting campaigners to push for a right to refer them to domestic courts.
Pressure is growing on the UK after the US concluded that China is committing genocide – but ministers are determined to preserve their post-Brexit freedoms.
Boris Johnson’s official spokesperson said: “It is common practice during 'ping pong' on Bills for some Lords amendments to be packaged together where they cover similar issues".
And Greg Hands, the trade minister, claimed: “Genocide is notoriously hard to prove with a high legal threshold.
“If a judge was unable to make a preliminary determination on genocide, which is highly probable, it would be a huge propaganda win for the country in question – effectively allowing that state to claim it had been cleared by the UK courts.”
But Ms Ghani retorted: “We should never believe that because a case may fail that people won't put a case forward to court. That's just a nonsense.”
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