UNITED STATES SECURITIES AND EXCHANGE COMMISSION Washington, D.C. 20549 FORM 6-K REPORT OF FOREIGN PRIVATE ISSUER PURSUANT TO RULE 13a-16 OR 15d-16 UNDER THE SECURITIES EXCHANGE ACT OF 1934 For the month of December, 2005 SPIRENT plc _____________________________________________________________________ (Translation of registrant's name into English) Spirent House, Crawley Business Quarter, Fleming Way, Crawley, West Sussex RH10 9QL, UK. _____________________________________________________________________ (Address of principal executive offices) Indicate by check mark whether the registrant files or will file annual reports under cover Form 20-F or Form 40-F. Form 20-F X Form 40-F..... Indicate by check mark whether the registrant by furnishing the information contained in this Form is also thereby furnishing the information to the Commission pursuant to Rule 12g3-2(b) under the Securities Exchange Act of 1934. Yes ..... No X If "Yes" is marked, indicate below the file number assigned to the registrant in connection with Rule 12g3-2(b): 82- ________ NOTIFICATION OF MAJOR INTERESTS IN SHARES 1. Name of listed company SPIRENT PLC 2. Name of shareholder with a major interest FIDELITY INTERNATIONAL LIMITED 3. Please state whether notification indicates that regarding the holding of the shareholder named in 2 above; in respect of a non-beneficial interest; or in the case of an individual holder if it is a holding of that person's spouse or children under the age of 18 SHAREHOLDER IN 2 ABOVE AND ITS DIRECT AND INDIRECT SUBSIDIARIES, BEING NON- BENEFICIAL HOLDERS 4. Name of the registered holder(s) and, if more than one holder, the number of shares held by each of them JP MORGAN BOURNEMOUTH (FPM) 4,551,093 NORTHERN TRUST LONDON (FPM) 3,893,100 MELLON BANK (FPM) 1,483,900 BANK OF NEW YORK BRUSSELS (FPM) 1,232,100 STATE STR BK AND TR CO LNDN (FPM) 1,055,400 BANKERS TRUST LONDON (FPM) 823,500 MIDLAND SECURITIES SERVICES (FPM) 371,200 CLYDESDALE BANK PLC (FPM) 118,700 JP MORGAN BOURNEMOUTH (FISL) 24,422,687 BROWN BROS HARRIMN LTD LUX (FIL) 35,038,433 BANK OF NEW YORK BRUSSELS (FIL) 415,700 NORTHERN TRUST LONDON (FIL) 136,100 JP MORGAN, BOURNEMOUTH (FII) 1,395,500 BNP PARIBAS, PARIS (FIGEST) 521,500 TOTAL 75,458,913 FPM = Fidelity Pension Management FISL = Fidelity Investment Services Limited FIL = Fidelity International Limited FII = Fidelity Investments International FIGEST = Fidelity Gestion 5. Number of shares / amount of stock acquired - 6. Percentage of issued class (any treasury shares held by the listed company should not be taken into account when calculating percentage) - 7. Number of shares / amount of stock disposed NOT KNOWN 8. Percentage of issued class (any treasury shares held by the listed company should not be taken into account when calculating percentage) NOT KNOWN 9. Class of security ORDINARY SHARES OF 3.3333 PENCE EACH 10. Date of transaction NOT KNOWN 11. Date company informed 16 DECEMBER 2005 12. Total holding following this notification 75,458,913 13. Total percentage holding of issued class following this notification (any treasury shares held by the listed company should not be taken into account when calculating percentage) 7.81% 14. Any additional information ON 09 DECEMBER 2005 THE SHAREHOLDER REPORTED TO THE COMPANY THAT IT HAD INCREASED ITS HOLDING IN THE COMPANY TO 97,485,796 SHARES (10.09%). 15. Name of contact and telephone number for queries MICHAEL ANSCOMBE - 01293 767672 16. Name and signature of duly authorised company official of the listed company responsible for making this notification MICHAEL ANSCOMBE - ASSISTANT COMPANY SECRETARY Date of notification 19 DECEMBER 2005 Signatures Pursuant to the requirements of the Securities Exchange Act of 1934, the registrant has duly caused this report to be signed on its behalf by the undersigned, thereunto duly authorized. ______LUKE THOMAS______ (Registrant) Date 19 December, 2005 By ____/s/ Luke Thomas____ (Signature)*