The EU said on Thursday that growth in Britain had "slowed markedly" in 2017 and was projected to fall to 1.5 percent for the year, down from its previous estimate of 1.8 percent.
Brussels believes that the British economy will slow further to 1.3 percent in 2018 and 1.1 percent in 2019.
The figures put Britain's economy lowest in the bloc with Italy, and far behind the growth rate predicted for the euro area, which is expected to be 2.2 percent this year and 2.1 percent in 2018.
"Economic growth in the UK has been slowing since the start of the year, as higher consumer prices constrained private consumption growth," the EU forecast said.
"Based on a purely technical assumption of status quo in terms of trading relations between the EU27 and the UK, growth is still expected to remain subdued over the forecast horizon."
EU officials decided to include a UK forecast for 2019 despite the uncertainty over what Britain’s trading relationship will be with the rest of the bloc after it departures the EU in March 2019.
The figures were released as a sixth round of the slow-moving Brexit negotiations began in Brussels.
British Prime Minister Theresa May acknowledged in October that Brexit negotiations with the EU have hit “difficulty.
EU leaders agreed at a summit last month that not enough progress had been made in negotiations over EU citizens' rights, the border in Northern Ireland and the UK's financial obligation to begin formal trade talks.
EU leaders have been increasingly frustrated about divisions in May's cabinet over Brexit, saying they are still unsure what the UK wants, even after five rounds of negotiations.