HMS Mersey is filling the gap until two Border Force cutters return from the European Union's Mediterranean migrant crossings mission.

Her Majesty's Ship Mersey, an offshore patrol vessel with a crew of around 45, left the Channel port of Portsmouth on Thursday.

Attempts to get to Britain by crossing the world's busiest shipping lane in tiny craft have surged in the last three months, with numbers spiking over the Christmas holidays.

"HMS Mersey will deploy to the Dover Straits to assist the UK Border Force and French authorities with their response to migrant crossings," said Britain's Defence Secretary Gavin Williamson.

"The crew have been able to immediately divert from routine operations to help prevent migrants from making the dangerous journey across the Channel."

The navy ship will help "keep the UK border secure and prevent loss of life", the Home Office interior ministry said.

Two coastal patrol vessels and two Border Force cutters are already deployed in the Channel.

Any migrants found in British waters are taken to a UK port.

Britain will keep one cutter on the EU Mediterranean mission and bring two back: Protector, which is anchored at the Greek island of Lesbos, and Seeker, which is docked in the overseas territory of Gibraltar.

Meanwhile, detectives are continuing to question two men arrested on suspicion of arranging the illegal movement of migrants into Britain.

A 33-year-old Iranian national and a 24-year-old British man, arrested in Manchester on Wednesday, remain in police custody.

Some 539 people crossed the Dover Straits -- the Channel's narrowest part at 21 miles (33 kilometres) wide -- in 2018, 80 percent making the journey in the last three months.

Almost all those who have made it to Britain have requested asylum, Home Secretary Sajid Javid said Wednesday, but he questioned whether someone who had left the safety of France could be a "genuine asylum seeker".

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