Shares in Bumi, the FTSE 250 coal miner, lost a third of their value on Monday after it announced an inquiry into potential financial irregularities at its Indonesian unit.

The company said an independent investigation was already established into allegations centred on its PT Bumi Resources arm, in which it holds a 29 percent stake. The investigation will include scrutiny of what the company called “the development funds of PT Bumi Resources Tbk”, which were marked down to zero in its accounts running to December 31, 2011.

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The news sent shares in the company falling 33 percent to 130p, the biggest single loss on London’s second-tier index by a clear margin.

The FTSE 250 also had the session’s most eye-catching gains.

Petra Diamonds rose 6 percent to 116.4p after it issued an upbeat production forecast alongside news of a 3 percent rise in annual profit from it mining operations of USD 103.3m. It was the best single gain on the index.

Qinetiq rose 4.7 percent to 180p after it said its interim numbers would be stronger than expected. The defence technology company reported stronger-than-forecast interim numbers, helping it stand by existing profit guidance for the full year even in the context of defence spending cuts in its main markets, especially the US.

Andrew Gollan, analyst at Investec, raised his target price on the stock from 130p to 150p and said: “The board’s full-year expectations will at least be met, which we interpret translates to a 10 percent uplift to consensus full-year earnings targets for 2013.

“Visibility beyond the second half remains poor and hence we do not anticipate materially changing our estimates thereafter.”

Overall, both of London’s main indices remained unsettled, with the rally inspired by the late summer run of action from central banks looking all the more like it had run its course.

The FTSE 100 fell another 29 points to 5,823.94, and was once again left nearer 5,800 than the valuation over 5,900 reached at the peak of the rally.

“After reaching a top at 5,932.60 on September 14, the FTSE 100 . . . has turned the main trend to down and could be poised for an acceleration to the downside over the near term,” said James A. Hyerczyk, analyst at Autochartist.

“Based on the rally from 5,634.90 to 5,932.60, traders should look for a possible test of 5,783.75 to 5,748.62 over the near term.”

Heavily-weighted resource stocks led the selling once again, with Russian steelmaker Evraz the biggest single faller, down 3.1 percent at 263.6p. Nine of the 10 biggest fallers on the index were from the sector, as traders across global markets were left unconvinced about the outlook for global economic growth.

The FTSE 250, seen as more representative of the domestic UK economy, was 77 points weaker at 11,873.29, a fall of 0.6 percent. Its losses were led by investment managers and the construction sector.