The Foreign Affairs Select Committee has called for William Hague's department to adopt a "more realistic approach" to what could be achieved given recent cuts.
The MPs also raised concern about the future funding of the BBC World Service and strongly opposed plans for its wider commercialisation.
They were told a planned 10% cut in the number of UK-based staff at the FCO would not be met by the original target of 2015.
The FCO was on course to make savings of about £170 million over the Spending Review period to 2014/15 but warned of the effect of this.
The committee's report said: "There are signs that the FCO is being stretched, almost to the limit. The department will need an extra year to meet its target of a 10% headcount reduction in UK-based staff, largely because of 'sustained pressure to deliver an ambitious policy agenda'.
"A more realistic approach by the department and by ministers is needed."