An Iranian and a British national have been arrested on suspicion of facilitating illegal migration across the Channel, the world's busiest shipping lane.

Attempts to get to Britain aboard small craft have surged in the last three months, with numbers spiking over the Christmas holidays.

The offshore patrol vessel HMS Mersey -- currently in the Channel port of Portsmouth -- was "available and ready" to be deployed, a Ministry of Defence source told the domestic Press Association news agency.

An MoD spokesman told AFP: "Our armed forces stand ready to provide additional capacity and expertise to assist the Home Office with the response to migrant crossings.

"Royal Navy ships continue to conduct patrols to protect the integrity of UK territorial waters."

Some 539 people crossed the Strait of Dover -- the Channel's narrowest part at 21 miles (33 kilometres) wide -- in 2018. Eighty percent maade the journey in the last three months, said Home Secretary Sajid Javid, the interior minister.

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

He immediately faced a barrage of criticism from opposition MPs.

Javid said any migrants picked up in British waters would be taken to a UK port.

Meanwhile a 33-year-old Iranian national and a 24-year-old British man were arrested in Manchester, northwest England, on Wednesday "on suspicion of arranging the illegal movement of migrants across the English Channel into the UK", a National Crime Agency spokeswoman said.

The investigation is ongoing.

Britain is redeploying two Border Force cutters from the Mediterranean in response to the situation.

The Sunday Times newspaper, reporting from a migrant camp on the French coast, said a growing number of well-educated Iranians were attempting the Channel crossing.

It cited Ali, 34, a car salesman from Tehran, as saying his journey from the Iranian capital to Britain would end up costing pound15,000 ($18,900, 16,600 euros), with payments in stages to a Kurd-dominated people smuggling gang.