Under a general partnership a partner cannot retire or leave. The partnership has to be dissolved. One partner can dissolve the partnership by giving notice to the others partners.
The remaining partners can then reform the partnership, but there are negative tax implications to this. If there is a partnership agreement (or you are an LLP) then it is possible for the partnership to continue. Agreements need to be reached as to limiting the partner's liability to the time that he or she was in partnership. Notices are placed in local papers and in the London Gazette to cover this point.
If you have a partnership agreement you need to look at the terms of that agreement to see how to exit.
Partnerships, where there is not a partnership agreement to the contrary, are also automatically dissolved on the death or bankruptcy of one of the partners. They also end if the purpose of the partnership is illegal.
At Ralli Partnership Law we have extensive experience of working for partners on the exit or dissolution of partnerships. Please contact us for a free consultation.