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There are more worries for the Prime Minister as his top aide on Brexit has resigned.

Lord Frost was the man who negotiated the deals that led to the UK leaving the European Union (EU) - following a referendum in 2016 - and agreed what their relationship would be going forward.

Now according to ����ý experts, he's decided to leave the government because he's not happy with the choices the Boris Johnson is making.

It's the latest in a series of difficulties for the Prime Minister and affects one of his most important projects.

Who is Lord Frost?

Lord David Frost is a former diplomat and UK Ambassador who joined Boris Johnson's team when he was Foreign Secretary under Prime Minister Theresa May.

He was the United Kingdom's Chief Negotiator for Exiting the European Union during the Brexit negotiations in 2019.

After the the UK left the European Union in January 2020, Lord Frost led the UK's negotiations with the EU on an agreement for their future working arrangements.

Since then, Lord Frost has been a member of the UK government, while also working with the EU on issues such as problems over the part of the Brexit deal which applied to Northern Ireland.

Why did he leave?

In his resignation letter to Mr Johnson, Lord Frost said that while "Brexit is now secure" he had "concerns about the current direction of travel".

����ý politics experts also say that Lord Frost is said to have been unhappy with the broader political direction of Boris Johnson's government and how he has been handling coronavirus restrictions.

Responding to Lord Frost, Mr Johnson said he should be "immensely proud of your historic service to this government and this country".

Why does it matter?

Lord Frost was known to be a tough negotiator over Brexit and had the backing of many in Mr Johnson's Conservative Party.

He is credited with drawing up the Brexit agreement which helped Mr Johnson to become win a general election in 2019.

His departure will unsettle many in the Conservative Party at what's already a tough time for the Prime Minister.

In the last few weeks, Mr Johnson has seen many of his own team vote against him over new Covid rules, as well as arguments over reports of Christmas parties in 2020 which could have broken restrictions that everyone else was being told to stick to.

And earlier this week, his party also lost an MP election it would have expected to win.

Who's taking over the role?

Foreign Secretary Liz Truss will replace Lord Frost as the UK's lead negotiator with the EU in post-Brexit talks.

She will retain her role as foreign minister alongside the new post.