Fifa: Team GB will not compete at Rio unless all home nations agree
Britain will not be able to field football teams at the 2016 Rio Olympics without Fifa's permission.
Fifa vice-president Jim Boyce said he has been given "an absolute categorical reassurance" that there needs to be agreement from all home nations.
The English Football Association wants to enter a men's and women's side, but the other home nations are against it.
Scotland feels a united British team could risk its independence within Fifa, world football's governing body.
Britain competed in the Olympic football tournament at London 2012, with both teams reaching the quarter-finals.
But it was thought to be a one-off, with the FA appearing to rule out entering teams again at Rio.
However, it emerged earlier this month that the FA had written to its counterparts in Scotland, Northern Ireland and Wales to ask them if they want to take part.
Football Association of Wales president Trefor Lloyd Hughes, who was "livid" with the plans when he first heard them, said he was encouraged by Fifa's stance.
As for the Scottish FA, it said England had "underestimated the strength of feelings from the other home nations".
Despite heated opposition from the Scottish, Welsh and Northern Irish associations, five Welshmen were selected for the men's squad in 2012, with two Scots making the women's.