Mammoth task of unpicking 40-year EU relationship may be Britain's most complex
By Gavin Cordon, Press Association Whitehall Editor
The negotiations on the UK's withdrawal from the European Union may be the most complex ever undertaken by a British government.
When they finally gather round the table in Brussels, the two sides will not only have to unpick a relationship dating back more than 40 years, they will also be attempting to negotiate an ambitious free trade agreement.
One of the first issues to be discussed will be the position of the 3.2 million EU nationals living in the UK and 1.2 million British citizens who have made their home in the EU.
On the face of it, it seems straightforward enough with both the British Government and the European Commission both declaring they want an early reciprocal deal to secure their future rights, marking an amicable start to proceedings.
However, even before the talks have begun, the European side has expressed concern that the British have underestimated just what is required to make such an agreement work.
Commission president Jean-Claude Juncker was said to have reacted with incredulity at a suggestion by Theresa May when they met for dinner in Downing Street in April that an agreement could be wrapped up at the next EU summit in Brussels at the end of June.
Donald Tusk, the European Council president, has warned there must be "real guarantees" for EU nationals to continue to live, work and study in the UK and that a "serious British response" was needed if they were to make progress.
Also high on the agenda will be the position of Northern Ireland and its border with the Republic.
With the Government having said it intends to withdraw from the European customs union, it is acknowledged "flexible and imaginative solutions" will be required to avoid a return to the "hard border" of the past, potentially undermining the Good Friday Agreement and the peace process.
Then there is the thorny issue of money. Although there has been no official figure from Brussels, reports suggest that it will be looking for a "divorce bill" of £50 billion - or higher - in settlement of the UK's outstanding liabilities.
Ministers have vigorously disputed the figure, insisting the final bill will be nothing like that amount. However they do acknowledge that the UK will have to honour its obligations and the stage is set for some serious wrangling over what that will come to.
Other issues to be discussed include future arrangements on security co-operation and intelligence sharing once Britain is outside the structures of the EU.
Once sufficient progress has been made on the withdrawal issues, the EU say they will open discussions on a new trade agreement.
Mrs May has said that she wants to end free movement of labour with the EU and has acknowledged that means the UK cannot remain in the single market.
Instead, she said that she wants to negotiate a "bold and ambitious" new trade agreement that replicates as far as possible the existing trade arrangements.
The EU has also said that it wants an ambitious agreement but at the same time it has made clear Britain cannot expect to enjoy all the benefits of membership from outside the bloc and that there will be a price to be paid for leaving