Theresa May's former legal chief Geoffrey Cox says it would be 'unconscionable' for Boris Johnson's government to over-ride the withdrawal agreement as Tory Brexit civil war grows

• MPs are preparing for tonight's vote on the controversial Internal Market Bill  
• PM says is needed to prevent EU holding Britain ransom over Northern Ireland
• Government admitted legislation will over-ride parts of Brexit deal from last year

Theresa May's former legal chief Geoffrey Cox said it would be 'unconscionable' for the Government to over-ride the Brexit divorce deal. 

MPs are preparing for tonight's vote on the controversial Internal Market Bill, which Prime Minister Boris Johnson says is needed to prevent the EU holding Britain to ransom over Northern Ireland. 

Ministers are braced for a rebellion by Tory MPs, who are alarmed by the Government's admission that the legislation will break international law by over-riding parts of the Brexit deal negotiated by the PM last year.

However, the majority of rebels are expected to hold fire until next week, when they will try to amend it to include a 'parliamentary lock' on powers that would breach the deal.

Mr Cox last night became the most prominent Tory MP to oppose the controversial bill. 

He said Mr Johnson should not 'observe treaty obligations with his fingers crossed behind his back', adding that he could not support a bill which risked undermining 'the standing and reputation of Britain in the world'.

Mr Cox – who was sacked from the Cabinet in February's reshuffle – wrote in The Times: 'It is unconscionable that this country, justly famous for its regard for the rule of law around the world, should act in such a way.'

Justice Secretary Robert Buckland yesterday defended the legislation, saying it was 'in accordance with the most honourable traditions of the British state'.

Mr Buckland has faced calls to quit, with critics saying the move is incompatible with his own oath as Lord Chancellor to uphold the law. 

He repeatedly ducked questions about his own position yesterday before finally answering: 'If I see the rule of law being broken in a way I find unacceptable then of course I will go.' 

It comes as Mr Johnson's chief Brexit negotiator was locked in an extraordinary public spat with his EU counterpart last night.

David Frost traded blows with Michel Barnier on social media when he denied threatening to block British food exports if trade talks collapsed.

Lord Frost said the EU negotiator 'explicitly' made the threat and warned it could lead to food from Great Britain being banned from sale in Northern Ireland.  

British negotiators have accused Brussels of threatening to block food exports worth £5 billion a year to the EU if there is no trade deal.

Mr Barnier yesterday said he was 'not refusing to list' Britain as a so-called 'third country' for food export purposes. But he said the listing could only take place when the UK explained its biosecurity rules.

In an exchange with Mr Barnier on Twitter last night, Lord Frost hit back: 'The EU knows perfectly well all the details of our food standards rules because we are operating EU rules. 

'It has been made clear to us in the current talks that there is no guarantee of listing us. I am afraid it has also been said to us explicitly in these talks that if we are not listed we will not be able to move food to Northern Ireland.'

Mr Barnier denied that the EU's position was a 'threat to the integrity of the UK', but added: 'We could not have been clearer about the consequences of Brexit.'

Tony Blair yesterday became the fourth former PM to criticise the Internal Market Bill, joining Theresa May, Gordon Brown and Sir John Major. 

In a joint article with Sir John, Mr Blair urged MPs to reject the 'shaming' legislation, saying it imperils the Irish peace process, trade negotiations and the UK's integrity.

But Business Minister Nadhim Zahawi said Lord Frost's revelations about EU tactics showed 'exactly why no minister, no responsible government can be a bystander and watch a part of the United Kingdom be harmed in this way'.

Tory veteran Sir Roger Gale confirmed he will vote against the second reading of the legislation tonight. He said: 'I am not a serial rebel, but I do have principles. If we enter into an international agreement, we have to stand by it.' 

Eau no! End of perfume bargains at duty-free

Duty-free bargains at airports will end on goods including perfume, clothing and electronics from January 1.

Ministers announced tax savings will now only apply to sales of alcohol and tobacco.

The decision, which affects all outbound passengers, has been called a 'hammer blow' to struggling airports. As much as 40 per cent of their income comes from airside retailers.

Industry experts say it could lead to thousands of job losses as shops pull out of airports.

They fear some regional airports could even go bust.

It has intensified calls for an airport Covid testing regime to re-open Britain's skies.

Karen Dee, chief executive of the Airport Operators Association, said: 'Passengers will be disincentivised from making purchases as they travel through the UK.

'Many foreign visitors will now choose to go elsewhere, attracted by the beneficial tax and excise regimes of our European competitors.'

Francois Bourienne, chair of the UK Travel Retail Forum, added: 'It may well be the best gift the UK could have given the EU as well as a massive blow for UK plc.'

The Treasury said the decision was taken 'as the tax concession was not always passed on to consumers in the airport'.