The charity was forced to defend itself after being labelled by Nigel Farage and others as a “taxi service for illegal trafficking gangs”.
It saw an outpouring of public support after revealing how its volunteers were receiving abuse and publishing harrowing footage of rescues.
The RNLI said it received over £200,000 in donations on Wednesday alone, a 2,000 per cent increase on the previous day and far above the daily average of £6,000 to £7,000.
It has also recorded a surge in the number of people viewing information on how to volunteer with the lifeboat charity.
Jayne George, the RNLI’s fundraising director, said: “We are overwhelmed with the huge level of support we have received from our amazing supporters in the last couple of days.
“We have seen an uplift in donations, with over £200,000 being donated yesterday alone through a combination of one off donations, new regular support and supporters increasing their regular donation amount. This is simply incredible.”
She said the charity had also seen a “small number” of previous supporters withdrawing their support over operations involving asylum seekers.
Ms George said the RNLI’s statements were not meant to be a fundraising campaign, adding: “We simply wanted to tell the story of our crews and make it clear that our charity exists to save lives at sea. Our mission is to save every one. Our supporters’ kindness means so much to us, without them we could not save lives at sea, every one is a lifesaver.”
Saijd Javid, the health secretary, was among high-profile figures saying he had made a donation on Wednesday and sharing the charity’s website on Twitter so others could do the same.
The RNLI has become a target for right-wing commentators and extremists over the past year, amid record numbers of small boat crossings over the English Channel.
Jayda Fransen, the former deputy leader of Britain First, has been among those calling for people to stop donations, while anti-migrant groups operating on the south coast have accused them of “escorting illegals” and “supporting the invasion”.
Mr Farage, the former Ukip leader, has repeated claims the charity was a “taxi service for illegal trafficking gangs”.
The RNLI has said its sole purpose is to save lives at sea, and that it does not discriminate between different kinds of incidents where people are at risk.
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The charity has no role in immigration enforcement or border control, and hands over responsibility for anyone rescued to relevant authorities when it reaches land.
Dominic Raab, the foreign secretary, said the RNLI had continued to operate within the law and do an “incredible job”.
He added: “At the same time, if we are talking about the wider small boats issue, that is something where, absolutely at the same time, we need to come down as hard as is humanly possible, working with our French partners.”
Priti Patel vowed to make small boat crossings “unviable” last summer, but the pledge was followed by record numbers that have continued to grow.
Last week, the government announced an agreement to more than double the number of police patrolling French beaches, with the UK to give France another £54m.
It follows several similar deals in recent years that have seen little success, and critics have questioned efforts to dissuade migrants with Facebook adverts carrying messages including “don't put your or your child's life in danger” and “we will return you”.
The UK has been unable to send any asylum seekers to EU nations they have passed through, including France, since 1 January because a key agreement expired with Brexit.
Several EU countries have told The Independent they would not sign any new commitment to take deported asylum seekers and that negotiations are not underway.
The Home Office has been pushing to jail asylum seekers who steer small boats as “people smugglers”, but prosecutions have been limited by new Crown Prosecution Service (CPS) guidance.
The Nationality and Borders Bill, if enacted, would criminalise any migrant who reaches the UK on small boats by creating a new offence of knowingly arriving in the UK without valid entry clearance.
It would also make it easier to prosecute people for steering boats or assisting asylum seekers who arrive via irregular routes.
It is not possible to claim asylum in the UK from abroad, or apply for entry clearance overseas with the purpose of claiming asylum.
According to Home Office figures, at least 2,000 migrants are currently massing around the Calais area with the hope of crossing the English Channel.
Overall asylum applications to the UK have fallen, but small boat crossings have risen dramatically after the coronavirus pandemic caused a decrease in the freight and passenger traffic.
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