LONDON (Reuters) - Below are the key excerpts from the resignation letter of Britain's ambassador to the European Union, Ivan Rogers, less than three months before London triggers complex talks on Britain's exit from the bloc.
The text of the letter was published by the BBC.
I am writing to you all on the first day back to tell you that I am today resigning as Permanent Representative.
As we look ahead to the likely timetable for the next few years, and with the invocation of Article 50 coming up shortly, it is obvious that it will be best if the top team in situ at the time that Article 50 is invoked remains there till the end of the process and can also see through the negotiations for any new deal between the UK and the EU27.
It would obviously make no sense for my role to change hands later this year.
I have therefore decided to step down now, having done everything that I could in the last six months to contribute my experience, expertise and address book to get the new team at political and official level under way.
This will permit a new appointee to be in place by the time Article 50 is invoked.
I know that this news will add, temporarily, to the uncertainty that I know, from our many discussions in the autumn, you are all feeling about the role of UKREP (the UK Representation to the EU) in the coming months and years of negotiations over "Brexit".
I am sorry about that, but I hope that it will help produce earlier and greater clarity on the role that UKREP should play.
My own view remains as it has always been. We do not yet know what the government will set as negotiating objectives for the UK's relationship with the EU after exit.
There is much we will not know until later this year about the political shape of the EU itself, and who the political protagonists in any negotiation with the UK will be.
But in any negotiation which addresses the new relationship, the technical expertise, the detailed knowledge of positions on the other side of the table - and the reasons for them, and the divisions amongst them - and the negotiating experience and savvy that the people in this building bring, make it essential for all parts of UKREP to be centrally involved in the negotiations if the UK is to achieve the best possible outcomes.
Serious multilateral negotiating experience is in short supply in Whitehall, and that is not the case in the (European) Commission or in the Council.
The government will only achieve the best for the country if it harnesses the best experience we have - a large proportion of which is concentrated in UKREP - and negotiates resolutely.
Senior ministers, who will decide on our positions, issue by issue, also need from you detailed, unvarnished - even where this is uncomfortable - and nuanced understanding of the views, interests and incentives of the other 27.
The structure of the UK's negotiating team and the allocation of roles and responsibilities to support that team, needs rapid resolution.
The working methods which enable the team in London and Brussels to function seamlessly need also to be strengthened.
The great strength of the UK system - at least as it has been perceived by all others in the EU - has always been its unique combination of policy depth, expertise and coherence, message co-ordination and discipline, and the ability to negotiate with skill and determination.
UKREP has always been key to all of that. We shall need it more than ever in the years ahead.
As I have argued consistently at every level since June, many opportunities for the UK in the future will derive from the mere fact of having left and being free to take a different path.
But others will depend entirely on the precise shape of deals we can negotiate in the years ahead.
Contrary to the beliefs of some, free trade does not just happen when it is not thwarted by authorities: increasing market access to other markets and consumer choice in our own, depends on the deals, multilateral, plurilateral and bilateral that we strike, and the terms that we agree.
I shall advise my successor to continue to make these points.
Meanwhile, I would urge you all to stick with it, to keep on working at intensifying your links with opposite numbers in DEXEU (the Department for Exiting the EU) and line ministries and to keep on contributing your expertise to the policy-making process as negotiating objectives get drawn up.
The famed UKREP combination of immense creativity with realism ground in negotiating experience, is needed more than ever right now.
I hope you will continue to challenge ill-founded arguments and muddled thinking and that you will never be afraid to speak the truth to those in power.
I hope that you will support each other in those difficult moments where you have to deliver messages that are disagreeable to those who need to hear them.
I hope that you will continue to be interested in the views of others, even where you disagree with them, and in understanding why others act and think in the way that they do.
I hope that you will always provide the best advice and counsel you can to the politicians that our people have elected, and be proud of the essential role we play in the service of a great democracy.
Reporting by Estelle Shirbon; editing by Guy Faulconbridge