The European Central Bank has offered to help the EU redesign its financial transactions tax to avoid any ‘negative impact’ on market stability, highlighting official fears about the implementation of the levy.

Proposals by 11 eurozone countries for a “Robin Hood tax” on trading in bonds, shares and derivatives have run into strong opposition from the financial industry, which has warned they could dry up markets, increase costs substantially for investors and erode bank profits.

Publicly, the ECB has refused to take sides, pointing out it has no mandate in the field. But its offer to “engage constructively” in the design of the tax suggests that, privately, it has deep reservations about its impact on financial markets and the real economy.

Benoît Cœuré, ECB executive board member, told the Financial Times: “We’re willing to engage constructively with governments and the European Commission to ensure that the tax has no negative impact on financial stability.”

It is hoped that the FTT will raise €30bn-€35bn a year in revenues and encourage more responsible behaviour by bankers. But disputes in Brussels look certain to delay implementation past its January 1 2014 start date, and the ECB intervention will fuel expectations that the plan will be watered down significantly.

The ECB is looking especially carefully at the FTT’s impact on bond trading and the market for “repos”, or repurchase agreements, by which assets such as government bonds are sold temporarily for cash and are an essential source of daily liquidity in the financial system.

Last month, Jens Weidmann, president of Germany’s Bundesbank, warned that in its current form the FTT “would cause considerable harm to the repo market”. If it was not able to function correctly, central banks would “remain heavily involved in the redistribution of liquidity among banks long after the crisis is over”.

The ECB believes markets should efficiently “transmit” changes in interest rates to the real economy. It would almost certainly back exempting the repo market from the tax, as well as steps to ensure costs did not increase for businesses using derivatives when hedging conventional trades. Mr Cœuré’s comments will add to suspicions the ECB would prefer to have a limited UK-style stamp duty on equities.

The planned FTT ran into strong opposition last week at the annual meeting of the International Capital Market Association in Copenhagen. Hakan Wohlin, global head of debt origination at Deutsche Bank, told the FT that bond markets were a “phenomenally attractive” way of channelling savings into investment. The FTT would be like throwing sand into a Mercedes-Benz engine, he said. “The effect of that is not going to be very good.”

Richard Comotto, senior visiting fellow at the ICMA centre at Reading university, argued proponents of the FTT misunderstood the repo market. “The idea that everything short term is speculative is nonsense. People have to keep liquid assets in order to react to the uncertain situation.”

Godfried de Vidts, chairman of ICMA’s European repo council, said the market was “about a safer way of doing business” and “makes the market function in the way they have been designed to function by regulators”.