The European Commission is set to launch its emergency exit plan on Wednesday to defuse confusion in key areas if Britain leaves the bloc in March without reaching a transitional agreement.
The move comes amid rising uncertainty over whether British Prime Minister Teresa May will be able to get parliamentary approval for an agreement on breaking out of the bloc with Brussels.
Britain is due to leave the European Union within 100 days on March 29, 2019.
The measures proposed by UNHCR are expected to cover areas of priority, as Britain´s exit from the bloc without an agreement would have a wide impact on people and companies such as citizens´ rights, air travel and financial services, according to a preliminary document released last month.
"Only emergency measures will be taken in limited areas, since it is essential to protect the EU´s main interests.
There will be temporary and limited procedures and will be adopted unilaterally by the bloc.
If Britain disengaged from the bloc in an unregulated fashion, European Union rules would not be applied to Britain overnight.
The repercussions will be wide-ranging: changing the legal status of British citizens living in the bloc, and vice versa, ending the border arrangements for people and goods to pass quietly and disrupt commercial chains.
Among other things, UNHCR is expected to propose measures to reduce obstruction of air traffic, for example allowing British aircraft to fly over EU countries and land in EU countries.
The European Commission also urged member states last November to adopt a "generous approach" to British citizens living in them.
Brussels is likely to present its proposals, provided that Britain takes equal action.
