The British government has proposed a four-phase system for introducing customs checks on food products traveling across the Irish Sea, to start in October, the BBC has reported.

The status of Northern Ireland, politically part of the United Kingdom, but physically separate from the rest of Great Britain and sharing a border with European Union member state the Republic of Ireland, has been a stumbling block the whole way through the Brexit process, and was addressed in the final Brexit agreement by a document known as the Northern Ireland Protocol.

The Guardian newspaper reports that phase one, starting in October, would involve certification for fresh meat products, with phase 2 in January addressing dairy products, garden centre plants, seeds and wine.

The next stage would cover pet foods, fruit and vegetables, and finally what are described as "ambient" foods, including items that have a limited shelf life, and high-risk foods not of animal origin. The dates of implementation for these two would depend on the success of the first two.

The protocol created a trade border down the Irish Sea, bureaucratically isolating Northern Ireland from the rest of the UK, but its implementation remains a thorny issue, and when in March the UK took a unilateral decision to delay introducing new border processes, the EU began legal action.

One of the great challenges that needs resolving is the moving of food products across the Irish Sea, as, to avoid the reintroduction of a hard border with the Republic of Ireland, Northern Ireland has stayed part of the EU's single market for goods, unlike the rest of the UK. This means food products going from the British mainland to Northern Ireland needing to be checked at ports, even though Northern Ireland is part of the UK.

Any suggestion of Northern Ireland being treated differently from other parts of the UK is particularly contentious for members of Northern Ireland's majority Unionist community, whose political loyalty is to the UK.

Brexit tensions were identified as one of the causes behind rioting that took place in Northern Ireland this year, and just to make things more difficult, the province's Democratic Unionist Party, which supported the Conservative government through the early stages of the Brexit negotiation process, has just elected a new leader, Edwin Poots, who strongly opposes the Irish Sea border.

Meanwhile, Irish state broadcaster RTE says there is growing concern the British government will try to completely rewrite the protocol. Writing in the Daily Mail, David Frost, who negotiated the protocol as part of the Brexit deal, suggested the government would be willing to work without it.

He accused the EU of having a "purist" stance that put the "political, social, or economic fabric of life in Northern Ireland" at risk, and hinted at a willingness to take unilateral action again.

"We are responsible for protecting the peace and prosperity of everyone in Northern Ireland and we will continue to consider all our options for doing so," he wrote.