British Airways, Lastminute.com, ferry companies P&O and DFDS, and hotel chain Radisson Edwardian have pledged money and support in kind to VisitBritain's marketing campaign.
VisitBritain declined to disclose the value of the commitments it had been promised so far.
Other companies making contributions will be announced in due course.
The Government money is intended to boost tourism around the Royal Wedding on 29 April, The Queen’s Diamond Jubilee, and the London 2012 Olympic and Paralympic Games.
Through the campaign, VisitBritain hopes to increase overseas visitors by one million, increase visitor spending in the UK by £2bn and create 50,000 new jobs within the tourism industry over the four-year period.
Prime minister David Cameron said: "Last year, we challenged British businesses to help us put together the best tourism marketing campaign ever for Britain – something that will make us the must-see destination for visitors from new growth markets in Asia and Latin America, as well as core ones like Europe and the US.
"The response has been magnificent, with pledges of cash and kind [sic] from companies, that mean we are well on the way to our target."
In October, chancellor George Osborne cut the funding of tourism marketing by 34% over a four-year period, in the comprehensive spending review.
Sandie Dawe, chief executive, VisitBritain, said: "The excellent response we have received already shows the huge appetite that British companies have for working together to grow our overseas tourism business. Yet today represents only the start. The names announced so far are just the first raft of our 'founding' partners.
"There will be plenty of opportunity for more companies to sign up to the campaign – which will put Britain on the wish list of the fast growing Asian and Latin American markets, and revitalise our appeal in core markets such as near-Europe and the US."