The Prime Minister has been accused of breaching trust in the government at home and abroad, following steps to change the EU withdrawal agreement.
Boris Johnson has faced criticism from isles MP Alistair Carmichael, who has stepped into the re-ignited Brexit row.
Mr Carmichael has highlighted what was described last year as an “oven ready” Brexit deal during the general election campaign.
He has questioned how the deal can now be claimed by the PM to be infringing on the UK’s freedom.
Speaking in the House during an urgent debate on the issue, Mr Carmichael said:
“What authority do we have to criticise China for not keeping her side of the bargain under the Joint Declaration in relation to Hong Kong if we are seen to treat our own treaty obligations to the European Union in this way?”
Responding for the government, Secretary of State for Northern Ireland, Brandon Lewis said: “We have got specific issues through the Protocol that were always designed to be worked out through the Joint Committee.
“I think it is right that the government is taking reasonable, sensible and limited actions in order to make sure we have that certainty in January should the negotiations on the withdrawal agreement and free trade agreement not come to a satisfactory conclusion.”
Commenting after the exchange, Mr Carmichael said: “The Prime Minister is insulting every voter who backed his ‘oven-ready deal’ last year and sending a message to every country across the world that his government cannot be trusted to respect its deals.
“He is simultaneously breaching trust in our government at home and abroad.
“Tearing up an international treaty – and the Good Friday Agreement with it – risks irreparable harm to our country.
“Forget the EU – other countries will not make trade deals with Boris Johnson if they believe he has his fingers crossed behind his back like a duplicitous child. International cooperation relies on good faith.
“Whatever your views on Brexit itself, we face a chronic weakening of our influence on the world stage if we are not seen as a credible negotiator.
“Whether it is supporting human rights and democracy in China, mediating on conflicts in the Middle East or defending our interests in trade talks, it will be more difficult to be taken seriously if we take our promises so lightly.”