FILE PHOTO: Britain's Prime Minister Theresa May sits next to Britain's Foreign Secretary Boris Johnson as she holds the first Cabinet meeting following the general election at 10 Downing Street, in London June 12, 2017. REUTERS/Leon Neal/Pool

LONDON (Reuters) - British Prime Minister Theresa May has told senior allies she will sack her foreign minister if he seeks to undermine a Brexit negotiating position agreed by her cabinet on Friday, the Times newspaper reported without citing sources.