"Because of its specific institutional features, the ECB needs to be differentiated from the union's institutions," said Mr Trichet in his letter sent to Manuel Lobo Atunes, Portuguese Europe minister.

This special status was secured in the original draft EU constitution, which fell by the wayside after being rejected by French and Dutch voters two years ago.

But the clause did not make it into the outline for a new EU treaty agreed by EU leaders before the summer and set to be finalised by the end of the year.

Mr Trichet fears that if the current treaty outline remains the same, with the ECB listed along the commission and parliament as an institution, it will be subject to the same general rules as these institutions which work together and follow certain agreed goals and European values.

This could leave the bank legally open to pressure from EU leaders to follow certain more politcal goals.

Maintaining the bank's independence has become more of a public battle ground since French president Nicolas Sarkozy came to power earlier this summer.

Mr Sarkozy has made several comments indicating that he would like to curb the bank's independence by calling for greater political influence in monetary policy making.

Mr Sarkozy has also repeatedly criticised the strength of the euro as well as what he sees as the bank's too strong emphasis on inflation.

Late last month the bank publicly hit back at the French president.

"The President of the ECB repeats with gravity that any attempt to seek to influence... the ECB in the performance of its tasks' violates Article 108 of the EC Treaty and that therefore such declarations are not acceptable,'' said a spokesperson on behalf of Mr Trichet.