The European Central Bank has ordered Dutch banking group ING to reverse part of a recent move of some units to London, the Financieele Dagblad said on Monday.

This means the planned saving of several hundred million euros which the move was set to generate is now in doubt, the paper said.

The ECB’s decision means that perhaps half the traders who were moved to London in 2016 and 2017 will have to move back to the European mainland, the paper said. Amsterdam and Frankfurt are being mooted as a possible location but no decision has yet been taken.

The ECB has been studying how ready Europe’s financial institutions are to deal with Brexit and says that banks must be able to to continue to provided the same services once Britain has left the EU.

This means they must have enough personnel and capital in an EU country to offset any risk.

ING has moved its currency and bond trading to the UK and this means, post Brexit, that they will no longer have full access to the European financial markets. The bank’s risk managers are also based in London, the paper said.