Former Prime Ministers
The former Prime Minister is formally styled ‘The Most Honourable’ when he/she is a member of the Order of the Nation and ‘Mr’ when he/she is not.
(when he/she is a member of the Order of the Nation)
The Mode of Address in:
Writing:
                        The Most Honourable Edward Seaga, ON, PC
Address
Opening Salutation:
                        ‘Dear Mr Seaga
Closing:
           ……………………………………………………………………………..
Yours faithfully (formal usage)
Yours sincerely (semi-formal usage)
Signature’
When referring to him and his spouse jointly, they are formally styled, ‘The Most Honourable Edward Seaga and The Most Honourable Mrs. Seaga’.
When referring to Mrs. Seaga alone, she is formally styled, ‘The Most Honourable Mrs. Edward Seaga’ and not ‘The Most Honourable Mrs. Carla Seaga’
The widows/widowers of former Prime Ministers would also be referred to in the same manner.
Other examples
1)     The late Right Honourable Hugh Lawson Shearer is now formally styled, ‘The late Most Honourable Hugh Lawson Shearer, ON, OJ’ or when appropriate, ‘The late Most Honourable Hugh Shearer’
The style of address for his spouse is, ‘The Most Honourable Mrs. Hugh Shearer’. In her professional capacity, she is styled ‘Professor Denise Eldemire Shearer’
2)     The late Honourable Michael Manley is now formally referred to as ‘The late Most Honourable Michael Norman Manley, ON, OM, OCC; or when appropriate ‘The late Most Honourable Michael Manley’
His widow is now formally styled, ‘The Most Honourable Mrs. Michael Manley’ and NOT ‘The Most Honourable Mrs. Glynn Manley’
3)     The late Sir Donald Sangster is now formally referred to as ‘The late Most Honourable Sir Donald Sangster, ON, KCVO’.