Citibank ( Europe), the Dublin operation of the giant US bank,is subject to a comprehensive assessment by the ECB, signalling that it is likely to be subject to direct supervision from Frankfurt in future.

The ECB announced on Tuesday that Citibank ( Europe) was one of four banks on which it is undertaking an assessment, which got underway in March and is expected to be finished by November.

In a statement, the ECB said that all banks that become or are likely to become subject to direct ECB supervision undergo “ an initial comprehensive assessment” by the ECB, including an asset quality review and a stress test. It said that the ECB regularly conducts such exercises in regard to banks that have recently been classified as significant or that may become significant.

They are separate from the 2016 EU-wide stress tests, under which the ECB is assessing a sample of euro area banks as part of an annual programme.

The ECB took on direct supervision of significant euro zone banks at the start of this year, including AIB, Bank of Ireland, Permanent TSB and Ulster Bank.

Citibank recently consolidated control of its European retail operations in Dublin, which may have sparked the ECB move.

Up to now Citibank ( Europe) was regulated directly by the Central Bank. Citibank ( Europe) has assets of $26.6 billion, and reported pre-tax profits of $720 million last year.

The other banks being assessed by the ECB are Abanka in Slovenia, Rietumu Banka in Lativa and Banca Mediolanum in Italy.