(Adds comment from Alphawave chairman, detail on recent IPOs) LONDON, May 13 (Reuters) - Canadian semiconductor technology company Alphawave’s shares plunged as much as 21% on their London market debut on Thursday, shrinking its value by more than 600 million pounds ($842 million) within hours. The slump comes amid a broader tech stock selloff and only weeks after food delivery firm Deliveroo fell as much as 30% on its debut, potentially hurting the London market’s ambitions to attract young fast-growing companies. Toronto-based Alphawave had already compromised on pricing its initial public offering (IPO), selling shares worth 856 million pounds at the middle of a previously announced price range at 410 pence, giving it a valuation of 3.1 billion pounds. By 1040 GMT the shares were down 21% at 322 pence, reducing the company’s market capitalisation to 2.45 billion pounds. “The book was many times oversubscribed and had a real global investor base, and (the IPO) was the highlight of our professional careers,” said Alphawave chairman John Lofton Holt. “We won’t judge this on a couple of hours of conditional trading.” He said that 80% of the deal was taken up by fund managers who buy shares on a long-term basis, while half the book comprised British investors. “Through sheer bad luck, Alphawave IP is coming to market in a volatile week,” said AJ Bell Investment Director, Russ Mould. U.S. tech stocks have fallen in recent days, particularly after stronger-than-expected inflation data stoked worries over tighter monetary policy. Other European IPOs have also struggled to gain traction recently, with German online car trader Meinauto scrapping its planned stock market listing on Tuesday, citing “adverse” market conditions for high-growth companies. Alphawave’s performance will raise questions about London’s IPO pipeline, with the likes of fintech business Wise and life sciences company Oxford Nanopore expected later this year. Alphawave sold 360 million pounds in new shares while existing equity holders sold down shares worth 496 million pounds, listing about 28% of the business. BlackRock and Janus Henderson were cornerstone investors in the Alphawave float, subscribing to about $390 million and 85 million pounds ($120 million) of shares respectively. The banks running the deal were Barclays and JPMorgan. The U.S. lender was also one of the two global co-ordinators on Deliveroo’s listing. Alphawave licences its high-speed data transmission technology to chipmakers and receives a royalty on every chip produced. Other large initial public offerings on the London Stock Exchange so far this year include Darktrace, Dr Martens , Moonpig and Trustpilot. ($1 = 0.7114 pounds) (Reporting by Joice Alves and Abhinav Ramnarayan, Editing by Thyagaraju Adinarayan, David Goodman and Gabriela Baczynska) Our Standards: The Thomson Reuters Trust Principles.