MUMBAI, April 26 (Reuters) - India's antitrust regulator has ordered a probe into Swiss drugmaker Roche for allegedly using anti-competitive practices to restrict cheaper copies of a blockbuster cancer drug from reaching patients. Trastuzumab has been a mainstay of Roche's profit for years and brought in global sales of about $6.7 billion in 2016, but it has been challenged in the last three years by biosimilars which are sold at about a 25 percent discount to the original. India's Biocon and U.S. firm Mylan, which together sell biosimilars of the drug in over a dozen countries including India, filed a complaint with the Competition Commission of India (CCI) last year alleging Roche misled doctors and regulators to thwart competition to trastuzumab. India, with among the highest number of cancer patients in the world, is a big market for trastuzumab, which is indicated to treat certain forms of breast and gastric cancer. Roche first launched the drug in India in 2002 and sells it for about 75,000 rupees ($1,170) for a 440 mg vial. In an interim order released on Tuesday, the CCI said it found merit in Biocon and Mylan's arguments, and ordered its director general to conduct a "detailed investigation" and submit a report within 60 days. "Roche adheres to all applicable laws and regulations in countries where it operates," Roche spokeswoman Shilpika Das said in a statement. "We are fully committed to cooperating with the authorities in India." Biocon said it welcomed the CCI's order, which although not final, but the investigation could result in penalties, a lawyer at the Delhi High Court, who declined to be named, said. Roche, which has been engaged in a legal tussle with Biocon and Mylan since 2014, has the option of filing a petition to contest the order, the lawyer added. In their complaint, Biocon and Mylan allege Roche wrote to doctors, hospitals, and state and federal regulators, misleading them about the safety of efficacy of the biosimilar versions. "When seen collectively in the background of surrounding facts and circumstances, they only appear to be a part of the bigger plan/strategy of Roche Group to eliminate competition posed by biosimilars to Roche’s products in the relevant market," the CCI said in its order, made public on Tuesday.