Nearly 30,000 migrants crossed the Channel from mainland Europe to Britain in small boats in 2023, an annual drop of more than a third, according to government figures released on Monday.

But the unauthorised arrival of 29,437 people on the southeast English coast is still the second-largest yearly tally since officials began publishing the numbers in 2018.

The perilous journeys across one of the world's busiest shipping lanes have become a political headache for Britain's Conservative government, with Prime Minister Rishi Sunak vowing last year to "stop the boats".

One of five key pledges he made for 2023, the promise of reducing persistently high numbers of migrant arrivals could haunt the Tory leader as he attempts to win a general election due this year.

Sunak said last month that there was no "firm date" for meeting his pledge.

Responding to Monday's statistics, his Downing Street office pointed to a 36-percent reduction in small-boat arrivals last year, after a record 45,000 migrants made the journey in 2022.

It credited "robust action" targeting "small boat gangs" -- partly through a £480 million ($610 million) collaboration agreement with France -- alongside fast-track migrant return deals struck with countries such as Albania.

That prompted more than 24,000 deportations and 246 arrests for people smuggling in 2023, according to Downing Street.

The government said it had cleared a "legacy" backlog of 112,000 asylum applications that were made before 28 June 2022, and this was reducing the number of hotel rooms being used to house would-be refugees.

But critics have accused the government of rushing decisions on applications, knowing many cases will ultimately be decided on appeal.

"I am determined to end the burden of illegal migration on the British people," Sunak said.

"That is why we have taken action to stop the boats, return hotels to their local communities and deter those wanting to come here illegally from doing so," he claimed.

- 'Unreasonable' -

But the main opposition Labour party -- which has enjoyed double-digit poll leads for the duration of Sunak's nearly 15 months in power -- says he has failed to keep his promise and his immigration policy is in chaos.

The Conservatives had hoped to deter the Channel crossings by preventing all migrants arriving without prior authorisation from applying for asylum and sending some to Rwanda.

However the Rwanda policy remains stalled after the UK Supreme Court ruled that deporting migrants to the east African country was illegal under international law.

The cross-Channel journeys on small inflatable vessels, which are often overloaded and unseaworthy, have repeatedly proved deadly.

In November 2021, at least 27 people drowned when their dinghy capsized.

The UK government is also under pressure to reduce the far higher level of immigration via pre-authorised arrivals, with the number in the year to June 2023 estimated to be 672,000 more than the total who left.

The figures contradict pledges by the Tories -- in power since 2010 -- to reduce overall migration to Britain after Brexit.

Ministers have in the last year announced various measures aimed at lowering that annual tally by 300,000, including by preventing almost all international students from bringing family members with them.

That change came into force on Monday, with interior minister James Cleverly calling the previous practice of permitting dependents to come with foreign students "unreasonable".

He added that the new rules "will see migration falling rapidly by the tens of thousands and contribute to our overall strategy to prevent 300,000 people from coming to the UK".