The Northern Ireland Protocol - agreed in the Brexit deal - is designed to avoid customs checks along the Irish border. It means goods - including medicines - can flow freely between the Republic of Ireland and Northern Ireland.

       However, goods arriving from the rest of the UK are subject to control to ensure they comply with EU standards.

       Due to a "grace period", controls have not yet been imposed on the movement of products.

       The grace period was expected to expire in January but the UK Government has indefinitely extended it.

       Lord Frost and his EU counterpart, Maros Sefcovic, have met on several occasions in a bid to resolve the teething issues with the Protocol.

       Prime Minister Boris Johnson has said he will invoke Article 16 of the Protocol if a deal is not struck with Brussels.

       Despite the UK reportedly softening its stance, Downing Street has insisted the option to invoke Article 16 remained on the table.

       However, Emeritus Professor Adrian Guelke from Queen's University Belfast has said the Protocol has to be made to work as there is no alternative.

       He told Express.co.uk: "The Protocol has to be made to work because there is not an alternative.

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       "They have an alternative of a hard border and checks on the border but I think the immigration rules that are being suggested for Northern Ireland are very peculiar.

       "They are proposing for people to apply online to come into Northern Ireland.

       "I do not see how that could possibly work without there being any checks.

       "Why would people apply for something if there are not any checks? It looks like a hard border."

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       However, the idea of a hard border across the island would be "hugely problematic", the expert warned.

       He continued: "I think a hardening of the border is out of the question. It is hugely problematic.

       "It will certainly push nationalists towards demanding a border poll which would upset loyalists and create a potential for violence which will not be great.

       "It seems very counterproductive to stability in Northern Ireland.

       "So something like the Protocol has to be made to work because there is not an alternative."

       Professor Guelke went on to say how trade between Northern Ireland and the Republic is not very significant but said the idea of leaving the single market open is a "fantasy".

       He continued: "I don't see how you cannot have checks at the border without the Protocol.

       "The Protocol has to work, it is the only solution.

       "Unless the UK aligns itself closer to the EU.

       "If the UK was in the single market and customs union then the Protocol would be practically redundant."

       Despite negotiations over the Protocol have been going on for weeks, Professor Guelke suggested it is unlikely a solution will be made until next year.