LONDON - A family of three has arrived in Britain as the first people under the government's "one in, one out" agreement with France, the Home Office announced on Wednesday.

The arrival of the family, including a child, came after the removal of the first four migrants from Britain to France as part of the year-long scheme.

"This is a clear message to people-smuggling gangs that illegal entry into the UK will not be tolerated," said a Home Office spokesperson. "We will continue to detain and remove those who arrive by small boat."

The "one in, one out" scheme was announced in July by British Prime Minister Keir Starmer and French President Emmanuel Macron during the latter's state visit to Britain.

ALSO READ: British PM vows to detain, send back illegal migrants amid mounting pressure

Under the treaty, migrants arriving in Britain via small boats will be detained and returned to France. In exchange, Britain will accept the same number of individuals who arrive via a newly established legal route. Entry will be limited to those who meet strict eligibility criteria and have not previously attempted illegal crossings.

Last week, an Indian national was the first person to be removed from Britain under the scheme, followed days later by three other people of other nationalities.

More than 30,000 people have crossed the English Channel in small boats so far this year, according to data from the Home Office. It remains unclear how many of them will be sent back and whether the new scheme will serve as a strong deterrent.

Illegal immigration is one of the hardest issues facing the British government, sparking a wave of protests across the country since summer this year.