By Simon Zekaria

LONDON--Shares in Nasstar PLC (NASA.LN) sank Tuesday after the cloud-computing services company warned on yearly performance, but said it is trading in line with expectations for the six months to June 30.

Still, the second half of the year will see some "unexpected challenges" on some customer contracts, which will impact revenue, the company said without disclosing numbers, including a contract which loses licensing revenue as a result of a client's deal with U.S. software-computing giant Microsoft Corp. (MSFT)

Nasstar said it is managing the issues it faces.

"Strong cost and cash control continues and the initiatives in these areas in the first half of the year will help reduce the cumulative impact of these developments, any one of which in isolation would have been insignificant in the context of overall group performance," it said in a statement.

The company said it is confident the full-year results will show "good" year-over-year growth, without elaborating, but said results for the period are expected to fall short of company expectations.

At 0747 GMT, shares fell 12% to 7.4 pence, valuing the company at 31 million pounds ($48.3 million).

Write to Simon Zekaria at simon.zekaria@wsj.com

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