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Prudential has delayed publication of details of a $21bn (£13.8bn) rights issue intended to help fund its $35.5bn acquisition of AIA in Asia.

The insurer said it was continuing discussions with the Financial Services Authority about the terms of the deal.

The delay comes amid turmoil in world stock markets, and unease among investors at the size of the cash call.

Prudential's statement did not set a new date, but it said it was "entirely committed to the transaction".

The FSA declined to comment.

The company had been expected to release details to the stock market at 0700 BST on Wednesday.

However, the ´óÏó´«Ã½'s business editor, Robert Peston, said there was a last-minute intervention by the FSA on Tuesday night.

"I am told that the FSA raised concerns about the so-called capital structure of the Prudential as enlarged by the enormous takeover," Mr Peston said.

"Or to put it another way, the FSA remains to be convinced that the Pru will be strong enough in a financial sense when the deal has gone through."

Prudential chief executive Tidjane Thiam announced the record-breaking rights issue in March, having been in the job for less than a year.

The company was adamant that the bid for AIA, part of bailed-out US insurer AIG, would not be knocked off course by the FSA's intervention.

Pru chairman Harvey McGrath said: "We are entirely committed to the transaction and remain on track to complete within the timing set out on 1 March.

"The work completed since 1 March with the AIA and Prudential teams has convinced me more than ever that the enlarged Group will be in a position to capture sustainable and highly profitable growth and will deliver substantial long term value for our shareholders," he added.

Prudential added that discussions with other regulatory and anti-trust bodies was progressing as planned.

Prudential shareholders were reportedly already nervous about the size of the rights issue and the company's ability to raise the money at a time when investors are cautious about prospects for the world economy.

But Eamonn Flanagan, insurance analyst at Shore Capital, said: "It is too early to call the death of the deal.

"There is shareholder dissatisfaction, but until investors actually have something to vote on, it is too early to talk about the implications of this delay for the wider transaction."

Prudential needs approval from 75% of its shareholders for the deal to go through. A shareholder vote is scheduled for 27 May.

But, according to newspaper reports, Capital Research & Management, Prudential's largest shareholder with 12%, has misgivings about the AIA takeover

The deal is set to double the size of Prudential, and make it the biggest insurer outside China, pushing it ahead of major rivals AXA and Allianz.

The FSA telephoned Prudential at about 1930 BST on Tuesday, just as the insurer and its advisers were pricing the rights issue and finalising plans for a series of presentations to investors.

The regulator said it was still "stress testing" the enlarged group's solvency ratio - a measure of a company's ability to meet its long term debt obligations.

It is thought that the FSA wanted a solvency ratio of more than 200%.

Prudential, with surplus capital of £3.4bn, and AIA, with $3.3bn (£2.1bn), are among the best capitalised insurers in the world and both above 200%.

Prudential hopes, therefore, that the FSA's queries are more technical than deal-breaking, and that the issue can be resolved in a matter of days.

But, say analysts, if Prudential is forced to raise more money to satisfy FSA stress tests - either through disposals or from the debt markets - resolving the issue could take longer than a few days.