Appeals have been broadcast on TV and radio by the Disasters Emergency Committee (DEC), which brings together 13 major British-based charities.
The funds it raises will be spent on search-and-rescue operations, medical care, food, clean water, clothes and temporary shelter.
Airlines have converted passenger planes so they can carry more cargo to the Caribbean nation, where the 7.0-magnitude earthquake has killed tens of thousands of people.
A British Airways Boeing 747 took off from London's Heathrow Airport on Saturday morning, bound for Haiti's neighbouring country, the Dominican Republic.
Seats were removed from the economy-class cabin so water containers, purification equipment and pumps could be stored there.
The company has also pledged £300,000 from its Unicef Change for Good programme, plus the £250,000 cost of the plane and its fuel.
Virgin Atlantic sent an airliner carrying aid and medical personnel to Miami, with further flights bound for Orlando and Jamaica due to leave in the coming days.
The prime minister said it was "extraordinary" that so much money had been donated to the appeal in such a short time.
"The generosity of people's spirit, our willingness to give even when some people have so little to give, humbles me," he told a meeting in London.
The DEC appeal for victims of the earthquake in Haiti
The international development minister, Mike Foster, said people had "responded magnificently".
"I think the very harrowing scenes on the ground that come through, being broadcast through people's TV screens, bring it home to individuals just the sheer scale of disaster that the people of Haiti are suffering," he told BBC News.
"Any natural disaster on that scale, I think, will bring the best out of people."
A special televised appeal for the DEC was broadcast on Friday evening on BBC One and ITV1.
Separately, the government has allocated £2m to deal with "logistical problems" on the ground in Haiti, such as the provision of cars and lorries to distribute aid, and to improve communications links.
Ann Barnes, 59, is originally from Leigh-on-Sea, Essex
This page is best viewed in an up-to-date web browser with style sheets (CSS) enabled. While you will be able to view the content of this page in your current browser, you will not be able to get the full visual experience. Please consider upgrading your browser software or enabling style sheets (CSS) if you are able to do so.