The secret plan represents the first time that full EU embassies have been discussed seriously.

The "Embassies of the Union" would be controlled by a new EU diplomatic service created by the Lisbon Treaty.

The Daily Telegraph has seen a high-level Brussels document discussing plans for a "European External Action Service" (EEAS) which was proposed under the new EU Treaty, currently being ratified in Westminster.

Talks have so far remained behind closed doors. Officials fear political fallout over plans to implement the new Treaty before it has been fully ratified.

Working papers circulating in Brussels suggest that more than 160 EU offices around the world, including in member states, would become embassies.

The new service would rival established diplomatic services. Britain, with one of the world's largest, maintains 139 embassies and high commissions in capital cities.

Equally controversial is a proposal for EU ambassadors who would be accountable to the European Parliament.

"Parliament should aim for proper hearings of special representatives and ambassadorial nominees in the tradition of the US Congress for nominations of a clearly political nature," says the document.

Plans for the new foreign service have raised highly sensitive political issues by giving trappings of statehood to the EU and by fusing, for the first time, national diplomats with existing "eurocrats".

A vicious battle over who should control the diplomatic corps has broken out between national governments and the European Commission.

Countries such as Britain are alarmed that the EEAS, which is expected to take on some consular activities, would be a stepping stone to a single "supranational" euro-diplomatic service.

Meanwhile, Brussels officials fear that, if controlled by national governments, the new EEAS would draw power from "Community" bodies, such as the Commission, to inter-governmental institutions such as the Council of the EU, which represents member states.

"Any inter-governmentalism of policy areas under Community competence has to be avoided," states the confidential document.

"The EEAS will have to be in a specific way administratively connected to the European Commission."

The EEAS will number between 2,500 to 3,000 officials at its inception in January next year. It is then expected to grow to 7,000, or even up to 20,000, according to different estimates.

Britain, which loses its veto over the EEAS after it is created by a European summit decision expected in October, is expected to contribute around 20 to 30 senior diplomats to the EU service.

William Hague, the shadow foreign secretary, said yesterday: "As predicted the renamed EU Constitution is forming the basis of a power grab by the EU. It exposes Labour's stupidity in giving up the veto on an area key to Britain's interests."

A Foreign Office spokesman said: "The UK opposes and will argue against naming EEAS offices embassies.