Britain, France and Germany have agreed on a European Union defence policy, an issue which had caused concern and controversy in the US.
France and Germany originally wanted separate EU military HQ
The deal is due to be presented at the EU summit in Brussels later on Friday.
The US feared a separate EU military planning unit - first sought by France and Germany - would duplicate Nato.
Nato chief George Robertson welcomed the new deal, which means making Nato planning staff the first port of call before consulting EU staff.
A spokesman for British Prime Minister Tony Blair said the agreement had a British stamp on it, according to the BBC's political correspondent Mark Mardell.
In practice, that means it does not challenge Nato or annoy the Americans, said our correspondent.
Mr Blair is understood to have talked to US President George W Bush twice in recent days about the plans.
Lord Robertson indicated that the new proposal met Nato and US concerns that a separate European defence policy would undermine the Nato alliance.
"If it is clear this is not a permanent establishment but simply the reinforcement of national sectors, then I think not only could we live with that but it could be advantageous," he told reporters.
He was speaking as the Italian EU presidency said it would be presenting the plans to other EU leaders at the summit in Brussels.
The deal comes before a weekend of tough talks over the constitution
Originally France and Germany had called for a separate headquarters to plan joint military operations by European Union nations.
Now it is explicit that they will first be expected to use planning staff at Nato.
If that cannot be done, they would use existing British and French headquarters.
Only if that is not possible would they use the European Union's own military planning staff, who would liaise closely with Nato.
Tensions over the US-led war in Iraq still remain in Europe but the breakthrough over defence means the plans will form a key plank of the EU's role under the new constitution - if it is agreed.
European leaders begin their formal meeting in Brussels on Friday and the intense talks over the constitution are expected to last all weekend.