Britain's ramshackle exit from the European Union, with two ministers quitting the government on Monday, could damage economic growth in the euro zone, European Central Bank policymaker Ewald Nowotny said.

British Foreign Secretary Boris Johnson and Brexit minister David Davis departed over Prime Minister Theresa May's plans to leave the European Union, leaving the British leader's Brexit plans in ruins.

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The ECB Bank is following the fallout from Britain's efforts to negotiate its departure, Nowotny said, highlighting it as one of the three main threats to economic growth in the 19 countries which use the single European currency.

"For the eurozone, the Brexit constellation is something we look at," Nowotny told an event in Zurich just before news of Johnson's exit. "It is mainly a risk for the UK but it also poses risks and uncertainties for the eurozone."

Other potential problems included looming trade wars along with constraints on economic capacity which could lead to slower growth, he said.

The ECB expects euro zone economic growth of 2.1 per cent this year, easing to 1.9 per cent in 2019 and a 1.7 per cent rate in 2020.

"These lower growth rates do not mean an end to the good times," Nowotny said. "It means we have reached a plateau and can grow with capacity growth."

In this uncertain environment, the ECB would adopt a "wait and see" approach to any tightening of monetary policy after it keeps negative rates on hold through the summer of 2019, he added.

The ECB said last month it intended to end its €2.6 trillion bond purchase programme by the close of the year but said rates would stay unchanged at least until next summer, a wording that pushed back rate hike expectations.