Net migration from the EU to the UK has slumped to a six-year low, while non-EU migration is the highest in more than a decade, according to new data issued by the Office for National Statistics.

There were 74,000 more EU citizens who came to the UK than people leaving for other EU countries. This was the lowest estimate for EU net immigration since 2012 and the lowest immigration level since 2014.

Non-EU net migration was at its highest since 2004, with 248,000 more non-EU citizens arriving than departing, the ONS data shows.

The Conservative MP Phillip Lee, who resigned from the government over its Brexit strategy, said it was a sign of “Brexodus”. “EU nationals are the pillar which support fundamental public services in this country. We cannot sit back and watch vital public services like our NHS be damaged by Brexit. That’s not what leave voters were promised in 2016,” he said.

Net migration from the EU A8 countries, which joined the bloc in 2004 and include Poland and the Czech Republic, was minus 14,000 in the year to June. Net migration of A2 citizens (Romania and Bulgaria) almost halved to 34,000 in the two years since the referendum. This is the lowest level since 2014, when these countries got full access to the UK labour market.

Net migration of EU15 migrants – from the older EU member states such as Germany, Italy and Spain – fell from 84,000 in the year to June 2016 to 47,000 in the same period this year.

Madeleine Sumption, the director of the Migration Observatory at Oxford University, said: “EU migrants have been leaving in larger numbers since the referendum, and net inflows have greatly decreased. The lower value of the pound is likely to have made the UK a less attractive place to live and work and economic conditions in several of the top countries of origin for EU migrants have improved.”

The data suggests non-EU net migration is more than three times the level of EU net migration. Overall, net long-term international migration was 273,000. This is down from the record levels of about 330,000 two years ago, but still almost three times the government’s target of less than 100,000.

Jay Lindop, the director of the ONS’s centre for international migration, said: “Net migration continues to add to the population and has remained fairly stable since its peak in 2016. However, there are different patterns for EU and non-EU migration. Due to increasing numbers arriving for work and study, non-EU net migration is now at the highest level since 2004.”

The ONS report says the decline was driven by a decrease in migration from the A8 eastern European countries. Increases in non-EU immigration for work and study have been recorded in the most recent year, particularly for Asian citizens, it says.

Sumption urged caution about the non-EU data. “We have doubts about the accuracy of the non-EU net migration figures. Other sources do not support the idea that non-EU citizens are currently contributing so much to net migration,” she said.

The shadow home secretary, Diane Abbott, said: “The data reinforces the view that the government’s net migration target is reckless and foolish. The target has never once been met and non-EU migration alone far outstrips it.”

The figures come as the government prepares to publish its long-delayed proposals for the UK’s post-Brexit immigration system.