Why the internal market bill is controversial in Brexit-deal?

The last September, UK politics reopened arguments about the Brexit-deal by introducing an internal market bill which could potentially breach the international law. It was a bit sensational as former PMs, including David Cameron and Theresa May joined to condemn the bill.

BBC news: Fifth ex-PM speaks out against post-Brexit bill

David Cameron has become the fifth former prime minister to criticise a new bill attempting to override the Brexit withdrawal agreement.

No 10 says the Internal Market Bill was a "critical piece of legislation for the UK".

But Mr Cameron said he had "misgivings" over it and breaking an international treaty should be the "final resort".

Former Tory PMs Theresa May and Sir John Major, and Labour's Tony Blair and Gordon Brown have condemned the plan. 

Since then, most of the critics focused on the government's attempting to override the Brexit withdrawal deal and a possibility of breaching the international law. But it seems little is said about how the controversial bill could fail to abide the law.

It is a little patchy information below but it is basically about the Northern Ireland Protocol preventing a hard border between Northern Ireland and Irish Republic.

BBC news: Brexit: Ministers plan laws overriding part of withdrawal deal

Meanwhile, the government will publish its Internal Market Bill - designed to protect trade arrangements between the four parts of the UK - on Wednesday.

This could contradict the Northern Ireland Protocol, set up to prevent a hard border between Northern Ireland and the Irish Republic, which many fear could be detrimental to peace.

The protocol says Northern Ireland will follow some EU customs rules - meaning customs declarations for goods moving from Northern Ireland to Great Britain, as well as some new checks on goods going from Great Britain into Northern Ireland - after the transition period.

The Financial Times said the bill would "eliminate" the legal force of the Withdrawal Agreement, struck less than a year ago between the UK and EU, in areas including state aid and Northern Ireland customs.

On the other hand, what the new bill rules:

BBC news: What is the row over UK 'internal markets' all about?

Right now, the UK is part of the European single market, with jointly agreed regulations and standards right across the continent.

Post-Brexit, the UK government wants to continue to have a joint market across England, Scotland, Wales and Northern Ireland - the "internal market".

But instead of the rules and regulations around things like food and air quality and animal welfare being set in Brussels, now they have to be set closer to home - and there is a row over who should have the final say.

Many powers are set to be directly controlled by the Scottish, Welsh and Northern Irish administrations, in fields including food labelling, energy efficiency and support for farmers.

However, the UK government has said the devolved administrations will still have to accept goods and services from all other parts of the UK - even if they have set different standards locally. 

The latter implies that Northern Ireland will have to accept some goods and services from the other administrations of UK regardless of the different quality standards on goods and services. The former says Northern Ireland will follow some EU customs rules so as to prevent the hard border between them and Irish Republic. Those lead to a conclusion that the argument arises because Northern Ireland will have to accept the goods and services from the other parts of UK, which do not complying with EU custom rules to be applied. Having sorted out so far, this is a substance of the argument about the internal market bill.