The United Kingdom's prime minister, Boris Johnson, will visit Northern Ireland on Monday to meet leaders of the newly formed executive following a breakthrough that has ended years of political paralysis.

But his trip comes as a new report warns that complications over implementing Johnson's Brexit plan in Northern Ireland could see the UK government taken to the European Court of Justice.

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Northern Ireland's main parties on Saturday formed a power-sharing government, ending a three-year standoff that threatened a key part of the region's 1998 peace settlement.

Johnson will press public service reform on his visit, having promised a huge cash injection to help Northern Ireland fund services if it could get its devolved administration, known as the Assembly, up and running again.

"This is an historic time for the people of Northern Ireland," Johnson said in a statement before his visit.

"The next decade will be an incredible time of opportunity for Northern Ireland and the whole of the United Kingdom as we come together to unleash the potential of our four nations," he added.

But a new computer system to handle the special arrangements necessary for post-Brexit trade between Britain and Northern Ireland, as well as between the Republic of Ireland, an EU member, and Northern Ireland, part of the United Kingdom, cannot possibly be delivered by the end of the year - and that is just one of several obstacles to "completing Brexit" this year, the Institute for Government (IFG) think tank has warned.

Similar systems would be expected to take up to five years to design and implement, the IfG cited a former head of the UK's tax collection agency as saying.

Johnson has set a firm deadline of the end of 2020 for ending the "transition period" and breaking free from the world's largest trading bloc, enshrining the date in law as part of his Brexit legislation which heads for scrutiny in the House of Lords, the UK's upper parliamentary chamber, on Monday.

If the deal is not fully implemented by then, the IfG warns, the EU could take the UK government to court for non-compliance.

"The deal has the support of no Northern Irish political parties and it looks almost impossible to complete the practical changes, for government and business, by the end of the year," reads the report.

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"Failure to comply with the withdrawal agreement could see the European Commission begin infringement proceedings and the UK ending up at the ECJ."

On Monday, Johnson will meet First Minister Arlene Foster and Deputy First Minister Michelle O'Neill - and the three will also meet Johnson's Irish counterpart, Leo Varadkar, who is himself under domestic pressure to announce an election date.

Before a 1998 peace deal, Northern Ireland suffered three decades of sectarian violence between Irish nationalist militants seeking a united Ireland and pro-British loyalists defending the region's place in the United Kingdom.

The Good Friday Agreement established the Assembly - a devolved legislature with a power-sharing leadership which has administrative responsibility for the province and can make new laws in areas such as the economy, finance and healthcare.

That arrangement collapsed in 2017 when Sinn Fein, the largest Irish nationalist party, withdrew, saying it was not being treated equally by the pro-British Democratic Unionist Party.

Last week, the British and Irish governments helped broker a deal to restore the Assembly.