BRUSSELS (Reuters) – EU antitrust enforcers face an uphill battle in tackling tech giants abusing their dominance because of the difficulty of finding remedies, the EU’s budget watchdog said on Thursday in its first audit of the regulators.

The report by the European Court of Auditors comes as critics of Google GOOGL.O voiced frustration at what they say is ineffective enforcement of a series of EU rulings ordering it to stop favouring its own online services to the disadvantage of competitors.

Besides Google, European Competition Commissioner Margrethe Vestager is also investigating Amazon AMZN.O, Apple AAPL.O and Facebook FB.O.

“Although the Commission has taken a number of case decisions tackling challenges resulting from the digital economy, significant challenges remain to be resolved,” the watchdog said.

“For example, practices in digital markets can cause damage to consumers. However, it is difficult for the Commission to find appropriate remedies to tackle an apparent competition problem as determining consumer harm can be particularly complex.”

The ECA said the issue applies not just to internet companies but also businesses involved in digital innovation such as those in energy, telecommunications, financial services and transport.

It said part of the problem lies in EU antitrust rules which only allow enforcers to act after wrongdoing has been committed.

The Commission “needs to scale up market oversight to be fit for a more global and digital world. It needs to get better at proactively detecting infringements and select its investigations more judiciously,” said ECA’s Alex Brenninkmeijer who was in charge of the report.

EU antitrust officials however have swatted away arguments that they are not able to do their job well in digital cases.

“EU competition rules are flexible enough to deal with digital markets,” Commission Director General for competition Olivier Guersent told an online event on Wednesday.