Britain's new European Commissioner will spearhead EU efforts to tackle terrorism, organised crime and cyber-security, it has been announced.
Sir Julian King, a career diplomat, was nominated by ex-UK PM David Cameron to replace Lord Hill, who resigned after the Brexit vote.
Lord Hill had been in charge of the financial services portfolio.
Downing Street said it welcomed the commission's decision, saying security was a "vital issue" for the EU.
Sir Julian - who has worked in the diplomatic service for 30 years, including most recently as Britain's ambassador to France - must be approved by MEPs and EU officials before taking up the position.
A Downing Street spokesman said: "The UK will continue to fulfil our rights and obligations as a member state until we leave the EU and the prime minister has been clear that we will be an active player, so it is right that we should continue to have a Commissioner role.
"Security is a vital issue for all member states and co-operation across the EU can help to better protect us all from the range of threats we face."
European Commission President Jean-Claude Juncker - who made the appointment - said security was a "pressing challenge" and the wave of terror attacks in France, Germany and Brussels underlined the need for "swift progress".
By Adam Fleming, BBC political correspondent in Brussels
The joke in Brussels was that the UK would be awarded the plum job of looking after paper clips until Britain leaves the European Union at some point in the next few years.
No-one's joking now because Sir Julian has been awarded a heavyweight portfolio.
He will be put in charge of the EU's efforts to tackle terrorism and organised crime. He will grapple with what to do about fighters returning from Syria and Iraq, cyber-security and the spread of illegal weapons.
No doubt this will please PM Theresa May whose main contribution during the referendum campaign was to say that the EU was useful for fighting cross-border crime.
But look more closely at the job description issued by the Commission President Jean Claude Juncker.
Sir Julian will report to one of the vice presidents, Frans Timmermans, and "support" the work of the commissioner responsible for migration, Dimitris Avramopolous. And many of the powers in these areas are jealously guarded by the member states themselves.
And of course, he will have to go through a grilling by a committee of the European Parliament, when MEPs return from their holidays in September.
All 28 EU countries have a European Commissioner, with each one taking responsibility for a different policy area, from telecoms to fish.
If Sir Julian gets the green light, he will take up his post at a time when the UK's relations with the EU are at an all-time low following the decision to end its 43-year membership of the organisation, previously known as the European Community.
He would be expected to play an important role as a "conduit" between the UK and the other 27 member states during the talks over the UK's exit.
Sir Julian will report to a series of vice-presidents who form Mr Juncker's top team.
During a career spanning decades in the diplomatic service, Sir Julian held a number of senior positions in Brussels, including acting as a senior aide to a previous EU commissioner.
Previous UK commissioners have included high-profile political figures such as Lord Patten, Lord Kinnock and Lord Mandelson.