Article 10 NI protocol
The MPs focused on Article 10 of the protocol which deals with "state aid", or government subsidies.
The terms of the NI Protocol, it means Northern Ireland is still in the EU single market for goods and remains subject to EU subsidy rules for trade in goods.
However,
Article 10 also says the UK as a whole should follow EU rules if a UK-wide subsidy could have an impact on trade in goods between Northern Ireland and the European Union.
For example, if a UK-wide subsidy was to give NI firms an unfair advantage over other companies in the EU single market.
Essentially, the UK government believes there is a very high bar for UK-wide subsidies to be covered by Article 10 while the EU sets the bar much lower.
The MPs concluded: "The continued application of EU state aid rules under the protocol on Northern Ireland was always likely to be controversial.
"However, it is worrying that the extent to which such rules will continue to be binding on the UK under Article 10 of the protocol are still interpreted very differently by the government and the European Commission."
They said the boundaries of Article 10 are likely to only become clearer through practice and, most likely, legal challenges.
Still years of legal battles ahead before brexit is done.