Prime Minister Rishi Sunak has pledged to bring down legal migration and said last week that he was considering a range of options to reduce the high levels of arrivals, part of his promises before an election expected next year.

The interior ministry said the new measures, which target post-graduate students except those on research programmes, will help cut migration "substantially" and stop people from using student visas as a backdoor route to find work in Britain.

"We have seen an unprecedented rise in the number of student dependents being brought into the country with visas," interior minister Suella Braverman said in a statement.

"This is the fair thing to do to allow us to better protect our public services, while supporting the economy by allowing the students who contribute the most to keep coming here."

Current rules allow postgraduate students studying courses lasting nine months or longer to bring partners and children to Britain, but the government said the number of dependants had jumped eightfold since 2019, to 136,000 people last year.

The measures, which will be effective from January, come before Thursday's release of annual net migration estimates for 2022. For the year ending June 2022, net migration reached an all-time high of 504,000.

Numbers should fall to pre-pandemic levels in the medium term, Braverman added, without offering a firm target.

High levels of legal migration, and the additional pressures on struggling public services, has long dominated Britain's political discourse and was one of the major drivers for the Brexit referendum in 2016.

Britain will also remove the ability for international students to switch out of the student route into work routes before their studies have been completed, the interior ministry said.

Student visas accounted for the largest proportion of migration to the UK with 486,000 issued last year, it added.