LONDON (Reuters) - South African insurer Old Mutual plans as early as next week to detail more arguments in favor of its $5 billion-plus Skandia offer when it sends out an addition to its offer prospectus for the Swedish insurer, sources familiar with the situation said.

Skandia on November 15 posted above-consensus third-quarter earnings saying the result showed it had a strong independent future, while Old Mutual the previous day announced nine-month life insurance sales up 18 percent.

Old Mutual will also "inevitably" lower the acceptance threshold of its offer from 90 percent, the sources said. About 15 percent of Skandia's shareholders have rejected Old Mutual's bid as too low.

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