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SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

FORM 20-F    

(Mark One)

Commission file number: 001-14958

NATIONAL GRID PLC

(Exact name of Registrant as specified in its charter)

England and Wales

(Jurisdiction of incorporation or organization)

1-3 Strand, London WC2N 5EH, England

(Address of principal executive offices)

Alison Kay

011 44 20 7004 3000

Facsimile No. 011 44 20 7004 3004

Group General Counsel and Company Secretary

National Grid plc

1-3 Strand London WC2N 5EH, England

(Name, Telephone, E-mail and/or Facsimile number and Address of Company Contact Person)

Securities registered or to be registered pursuant to Section 12(b) of the Securities Exchange Act of 1934:

Securities registered or to be registered pursuant to Section 12(g) of the Securities Exchange Act of 1934: None.

Securities for which there is a reporting obligation pursuant to Section15(d) of the Securities Exchange Act of 1934: None.

The number of outstanding shares of each of the issuer’s classes of capital or common stock as of 31 March 2020 was

Ordinary Shares of 12 204/473 pence each    3,780,237,016

Indicate by check mark if the registrant is a well-known seasoned issuer, as defined in Rule 405 of the Securities Act: Yes þ No ¨

If this report is an annual or transition report, indicate by check mark if the registrant is not required to file reports pursuant to Section 13 or 15(d) of the Securities Exchange Act of 1934. Yes ¨ No þ

Note — Checking the box above will not relieve any registrant required to file reports pursuant to Section 13 or 15(d) of the Securities Exchange Act of 1934 from their obligations under those Sections.

Indicate by check mark whether the registrant (1) has filed all reports required to be filed by Section 13 or 15(d) of the Securities Exchange Act of 1934 during the preceding 12 months (or for such shorter period that the registrant was required to file such reports), and (2) has been subject to such filing requirements for the past 90 days: Yes þ No ¨

Indicate by check mark whether the registrant has submitted electronically and posted on its corporate website, if any, every Interactive Data File required to be submitted and posted pursuant to Rule 405 of Regulation S-T (§ 232.405 of this chapter) during the preceding 12 months (or for such shorter period that the registrant was required to submit and post such files): Yes þ No ¨

Indicate by check mark whether the registrant has submitted electronically every Interactive Data File required to be submitted pursuant to Rule 405 of Regulation S-T (§ 232.405 of this chapter) during the preceding 12 months (or for such shorter period that the registrant was required to submit such files).: Yes þ No ¨

Indicate by check mark whether the registrant is a large accelerated filer, an accelerated filer, a non-accelerated filer or an emerging growth company. See definition of “large accelerated filer” “accelerated filer” and “emerging growth company” in Rule 12b-2 of the Exchange Act.

If an emerging growth company that prepares its financial statements in accordance with U.S. GAAP, indicate by check mark if the registrant has elected not to use the extended transition period for complying with any new or revised financial accounting standards^† provided pursuant to Section 13(a) of the Exchange Act.  ¨

†The term “new or revised financial accounting standard” refers to any update issued by the Financial Accounting Standards Board to its Accounting Standards Codification after April 5, 2012.

Indicate by check mark which basis of accounting the registrant has used to prepare the financial statements included in this filing:

U.S. GAAP ¨

International Financial Reporting Standards as issued by the International Accounting Standards Board þ

Other ¨

If “Other” has been checked in response to the previous question, indicate by check mark which financial statement item the registrant has elected to follow. Item 17 ¨ Item 18 ¨

If this is an annual report, indicate by check mark whether the registrant is a shell company (as defined in Rule 12b-2 of the Exchange Act). Yes ¨ No þ

This constitutes the annual report on Form 20-F of National Grid plc (the “Company”) in accordance with the requirements of the US Securities and Exchange Commission (the “SEC”) for the year ended 31 March 2020 and is dated 25 June 2020. Details of events occurring subsequent to the approval of the annual report on 17 June 2020 are summarised in section “Further Information” which forms a part of this Form 20-F. The content of the Group’s website (www.nationalgrid.com/uk) should not be considered to form part of this annual report on Form 20-F.

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