Annual net migration to the United Kingdom reached a record high of 606,000 last year, official estimates showed on Thursday, heaping pressure on British Prime Minister Rishi Sunak who has pledged to bring numbers down.

The increase was driven by non-European Union nationals, including refugees under the British government's Ukraine visa schemes and people migrating for work and education, the Office for National Statistics (ONS) said.

High levels of legal migration has long dominated Britain's political discourse and the topic was one of the major drivers for the Brexit referendum in 2016.

For more than a decade successive Conservative-led governments have promised to cut migration - once targeting a net figure of less than 100,000.

But data from the Office for National Statistics published on Thursday showed a net 606,000 people came to Britain in the year ending December 2022.

"The main drivers of the increase were people coming to the UK from non-EU countries for work, study and for humanitarian purposes, including those arriving from Ukraine and Hong Kong," said Jay Lindop, Director of the Centre for International Migration at the Office for National Statistics.

Lindop said there was evidence suggesting that immigration had slowed in recent months.

Previous data covering the year ending June 2022 had shown a net figure of 504,000, this was revised upwards in the latest release, also to 606,000.

The leaders of the Brexit campaign had argued that leaving the European Union would give Britain greater control of its borders and many who voted to leave cited high migration and the pressure they said it put on public services as a factor in their decision.

But in recent years Britain has opened visa schemes for people in Ukraine and former colony Hong Kong, while companies in sectors such as engineering, construction and catering have urged the government to allow them to hire international staff.

Net migration to Britain in 2015, the year before the referendum, was 329,000.

Sunak has pledged to reduce legal migration without giving a target. Earlier this week the government said it would remove the right of some international students to bring family members into the country.

Sunak has also vowed to crack down on illegal migration after tens of thousands of people arrived on small boats across the channel in recent years. (Reporting by Sachin Ravikumar and Muvija M, editing by William James and Michael Holden)