His statement, indicating that he will not be seeking re-election, will be his first public announcement of his decision since his intention to go after two years in the job was revealed in the Independent on 17 July.
The announcement will be made before an unprecedented extraordinary meeting that members have called so that they can protest about the conduct of their affairs and the pounds 2bn worth of losses they are having to meet.
Members at the meeting, which is expected to be stormy, will be critical of the Lloyd's ruling body - the 28-strong council - and will demand further safeguards for their interests as well as financial help for their losses.
Members will be upset that Mr Coleridge has nominated his own successor, David Rowland, a leading insurance broker, who does not sit on the present Lloyd's council and has yet to be elected.
Mr Rowland, aged 59, headed a task force that proposed reforms for Lloyd's over the next five to seven years. The report was published in January of this year.
He is chairman of Sedgwick Group, the largest independent insurance broking company in the UK.
If he gains the chairmanship of Lloyd's, Mr Rowland will become the first salaried chairman of the market. Sir Jeremy Morse, the banker, has recommended among other constitutional reforms that the post of chairman at Lloyd's, at present unpaid, should be salaried.
Mr Rowland's salary of up to pounds 500,000 at Sedgwick is likely to be matched and he will relinquish his post at the insurance broker.
Professional underwriters at Lloyd's would prefer an underwriter as chairman rather than an insurance broker. Stephen Merrett, chairman of Merrett Holdings, one of the largest agency groups, is their favoured candidate.
Other professionals in the market are critical of Mr Rowland. They argue that his well-received report on Lloyd's future for the next five to seven years was largely prepared by McKinsey & Company, the international management consultants, at a cost of up to pounds 2m for Lloyd's.
There is a strong lobby encouraging a former Lloyd's chairman, Sir Peter Miller, to return. But he has told friends that he is unwilling to take on the job.
The questions of the succession at Lloyd's and the structure of the new council are likely to overshadow the angry members' own resolutions.
The issue will be further complicated in that the underwriting members are divided among themselves about what should be done.
The Association of Lloyd's Members, which claims to represent more than 8,000 individuals in the market, has urged its members to support its resolution of confidence in the council and other resolutions proposed by more extreme protesters.
Lloyd's intends to hold a postal ballot on all the resolutions.
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