(Adds detail, comments) MELBOURNE April 2 (Reuters) - Testing...

(Adds detail, comments)

MELBOURNE April 2 (Reuters) - Testing laboratory ALS Ltd (ALQ) said on Thursday an investigation found that about half the certificates it provided for export coal samples over the past decade had been manually altered to improve the quality of the coal.

The assay reports, going back to 2007, had been amended at four laboratories of the coal superintending and certification unit of ALS's Coal business in New South Wales and Queensland states, it said in a statement to the Australian stock exchange.

Australia, which is known for its high quality coal, is the world's biggest exporter of the fuel to markets like Japan, South Korea, China, Taiwan and India.

The ALS unit has about 40% of the market for testing export coal samples to ensure shipments meet quality standards agreed with buyers, according to industry estimates.

The issue came to light earlier this year after allegations in an unfair dismissal case that Australian miner TerraCom Ltd (TER) had worked with ALS's Brisbane-based testing laboratory to falsely upgrade the quality of its coal in export documentation. TerraCom denied the allegations.

ALS, which launched an independent investigation into the claims, said on Thursday the probe had found no evidence of bribery or other third-party payments to ALS staff. It did not give a reason for the alterations.

"This is a very serious breach of the ethical standards we expect from everyone who works for ALS, and we have taken strong action to put an end to the behaviour," Chief Executive Raj Naran said in a statement.

An ALS spokesman told Reuters the manual amendments had upgraded coal quality characteristics such as energy and moisture content.

ALS said the general manager in charge of its coal assaying business and three other employees were no longer with the firm, while "stringent additional measures" had been taken to stop amendments to certificates without proper justification.

The Coal business certification unit accounted for less than 3% of the company's earnings, it said, and an independent review had cleared its other testing and sampling units of similar practices.

ALS said it had referred the matter to New South Wales police, in line with its legal duties.

Shares in ALS fell as much as 8.6% before ending down 4.1%. The company said it has been in contact with customers of its coal certification unit and there has been no material change to the performance of the business.

A Terracom spokesman declined to comment on Thursday.