British finance minister George Osborne ruled himself out of the running to succeed Conservative Prime Minister David Cameron on Monday, citing his fierce advocacy of staying in the European Union.

The government is under pressure to fill a vacuum left when Cameron announced he would resign by October after Britain ignored his advice and voted to leave the 28-member bloc in last week's referendum.

"It isn't in my nature to do things by half-measure, and I fought the referendum campaign with everything I've got. I believed in this cause and fought hard for it," Osborne wrote in the Times newspaper.

"So it is clear that while I completely accept the result, I am not the person to provide the unity my party needs at this time."