UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

SCHEDULE 13G

CUSIP No.  909205106

1

Names of Reporting Persons
I.R.S Identification Nos. of Above Persons (entities only)
White Mountains Insurance Group, Ltd. (No. 94-2708455)

2

Check the Appropriate Box if a Member of a Group (See Instructions)

(a)  o

(b)  o

3

SEC Use Only

4

Citizenship or Place of Organization
Bermuda

Number of
Shares
Beneficially
Owned by
Each
Reporting
Person with

5

Sole Voting Power
0

6

Shared Voting Power
1,798,100*

7

Sole Dispositive Power
0

8

Shared Dispositive Power
1,798,100*

9

Aggregate Amount Beneficially Owned by Each Reporting Person
1,798,100*

10

Check Box if the Aggregate Amount in Row (9) Excludes

Certain Shares (See Instructions)         o

11

Percent of Class Represented
by Amount in Row (9)
5.3%

12

Type of Reporting Person (See Instructions)
HC, CO

CUSIP No.  909205106

1.

Names of Reporting Persons
I.R.S. Identification Nos. of Above Persons (entities only)
White Mountains Advisors, LLC. (No. 04-6140276)

2.

Check the Appropriate Box if a Member of a Group (See Instructions)

(a)  o

(b)  o

3.

SEC Use Only

4.

Citizenship or Place of Organization
Delaware

Number of
Shares
Beneficially
Owned by
Each
Reporting
Person with

5.

Sole Voting Power
1,798,100**

6.

Shared Voting Power
0

7.

Sole Dispositive Power
1,798,100**

8.

Shared Dispositive Power
0

9.

Aggregate Amount Beneficially Owned by Each Reporting Person
1,798,100**

10.

Check Box if the Aggregate Amount in Row (9) Excludes

Certain Shares (See Instructions)         o

11.

Percent of Class Represented
by Amount in Row (9)
5.3%

12.

Type of Reporting Person (See Instructions)
IA, CO

Item 1.

(a)           Name of Issuer.

UniSource Energy Corporation.

(b)           Address of Issuer’s Principal Executive Offices.

One South Church Avenue, Suite 100, Tucson, AZ  85701

Item 2.

(a), (b), (c)              Name of Person Filing, Address of Principal Business Office and Citizenship.

This statement is being filed by White Mountains Insurance Group, Ltd., a Bermuda corporation (“WMIG”), and White Mountains Advisors, LLC., (“WMA”), a Delaware corporation (collectively the “Filing Persons”).  WMIG is a property and casualty insurance holding company and WMA is a registered investment advisor.

The address of the principal executive office of WMIG is 80 South Main Street, Hanover, NH 03755.  The address of the principal executive office of WMA is 370 Church Street, Guilford, CT 06437.

(d)           Title of Class of Securities.

Common Stock (no par value)

(e)           CUSIP Number.

909205106

Item 3.

If this statement is filed pursuant to §§240.13d-1(b) or 240.13d-2(b) or (c), check whether the person filing is a:

(a)

o

Broker or dealer registered under section 15 of the Act (15 U.S.C. 78o).

(b)

o

Bank as defined in section 3(a)(6) of the Act (15 U.S.C. 78c).

(c)

o

Insurance company as defined in section 3(a)(19) of the Act (15 U.S.C. 78c).

(d)

o

Investment company registered under section 8 of the Investment Company Act of 1940 (15 U.S.C 80a-8).

(e)

ý (1)

An investment adviser in accordance with §240.13d-1(b)(1)(ii)(E);

(f)

o

An employee benefit plan or endowment fund in accordance with §240.13d-1(b)(1)(ii)(F);

(g)

ý (2)

A parent holding company or control person in accordance with § 240.13d-1(b)(1)(ii)(G);

(h)

o

A savings associations as defined in Section 3(b) of the Federal Deposit Insurance Act (12 U.S.C. 18 13);

(i)

o

A church plan that is excluded from the definition of an investment company under section 3(c)(14) of the Investment Company Act of 1940 (15 U.S.C. 80a-3);

(j)

o

Group, in accordance with §240.13d-1(b)(1)(ii)(J).

Item 4.

Ownership

Provide the following information regarding the aggregate number and percentage of the

class of securities of the issuer identified in Item 1.

(a)

Amount beneficially owned:    1,798,100***  shares.

(b)

Percent of class:    5.3%.

(c)

Number of shares as to which the person has:

(i)

Sole power to vote or to direct the vote    0 .

(ii)

Shared power to vote or to direct the vote     1,798,100*** .

(iii)

Sole power to dispose or to direct the disposition of    0 .

(iv)

Shared power to dispose or to direct the disposition of    1,798,100*** .

***         The reporting person directly controls no UniSource Shares and indirectly controls, through various wholly-owned subsidiaries and certain of its employee benefit plans (as further described herein), 1,798,100 UniSource Shares.

Item 5.

Ownership of Five Percent or Less of a Class.

If this statement is being filed to report the fact that as of the date hereof the reporting person has ceased to be the beneficial owner of more than five percent of the class of securities, check the following  o.

Item 6.

Ownership of More than Five Percent on Behalf of Another Person.

Not applicable.

Item 7.

Identification and Classification of the Subsidiary Which Acquired the Security Being Reported on by the Parent Holding Company.

OneBeacon Insurance Company, The Camden Fire Insurance Association and Folksamerica Reinsurance Company are insurance companies as defined in Section 3(a)(19) of the Act (15 U.S.C. 78c).  The employee benefit plans sponsored by OneBeacon Insurance Company are employee benefit plans as defined in ‘240.13d-1(b)(1)(ii)(F).

Item 8.

Identification and Classification of Members of the Group.

Not applicable.

Item 9.

Notice of Dissolution of Group.

Not applicable.

Item 10.

Certification.

By signing below I certify that, to the best of my knowledge and belief, the securities referred to above were acquired and are held in the ordinary course of business and were not acquired and are not held for the purpose of or with the effect of changing or influencing the control of the issuer of the securities and were not acquired and are not held in connection with or as a participant in any transaction having that purpose or effect.

Date: February 13, 2004

WHITE MOUNTAINS INSURANCE GROUP, LTD.

by:

/s/

Name: J. Brian Palmer

Title: Chief Accounting Officer

WHITE MOUNTAINS ADVISORS, LLC.

by:

/s/

Name: Maribel C. Friend

Title:   Chief Operating Officer