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)*