LONDON, Oct 15 (Reuters) - Britain's top 350 companies must put out their book-keeping work to tender at least once a decade to increase competition in a market dominated by just four big accounting firms.

The Competition Commission published its final report on Tuesday following a probe into Britain's audit market but rowed back on an earlier draft recommendation that would force companies to re-tender their audit work every five years.

The watchdog had faced opposition from companies and the accounting sector's regulator, the Financial Reporting Council, who has just introduced a rule requiring firms to consider retendering at least once a decade.

The so-called Big Four accountants, KPMG, PwC, EY and Deloitte check the books of most blue chips across the world. Many top firms have kept the same accountant for decades.