The RNLI — the Royal National Lifeboat Institute — is a mostly volunteer service that rescues people in trouble off the British mainland.
Right-wing figures, including Nigel Farage, founder of the UK Independence Party which campaigned for Brexit, was one of the RNLI’s highest-profile critics.
RNLI chiefs said they had raised £200,000 on Wednesday, a 2,000 per cent increase on the previous day and far above the daily average of £6,000 to £7,000.
The RNLI began fighting back on Tuesday after attacks by Mr Farage, who accused the rescue service of being a “taxi service for illegal immigration".
RNLI fund-raising director Jayne George said the donors included new supporters, one-off payments and people increasing existing donations.
“We are overwhelmed with the huge level of support we have received in the last couple of days.
“This was never a fund-raising campaign — 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.”
This week, the RNLI released footage showing a rescue of a group of migrants on a small boat in the English Channel, reminding people of the role the RNLI plays.
More than 9,000 migrants have crossed the Channel so far this year, surpassing the total for 2020, when 8,417 people landed on British shores.
British Prime Minister Boris Johnson backs the “vital work” done by UK lifeboats saving lives at sea, his spokesman said.
Health Secretary Saijd Javid said he had made a donation while Dominic Raab, the foreign secretary, said the RNLI was doing an “incredible job”.
The government is trying to make it harder for migrants to cross the English Channel — the world’s busiest waterway — to reach Britain.