Mr Blair currently uses charter planes and the Queen's Flight
One is likely to be a long-haul plane with 70 seats, while a 15-seater jet will be ordered for shorter flights, BBC correspondent James Hardy says.
Since 1944 US presidents have had their own plane for presidential travel.
Although the Queen will have first call on the aircraft, they are already being dubbed "Blair Force Ones". The Lib Dems and Tories have criticised the plan.
The decision to order the two aircraft follows a study into the costs of ministerial travel by businessman and government spending adviser Sir Peter Gershon.
The planes will not be bought outright, but taken on a long lease and are unlikely to come into service until late 2007 at the earliest - by which time some think Mr Blair may have stepped down as PM.
VIP area
They are expected to cost around £12.3m a year - about £2.7m more than the current arrangement.
The flights are likely to be paid for by those who use them, like Number 10, the Ministry of Defence and the Royal Family - a similar arrangement to chartering and not just fuel and staffing costs.
It sends totally the wrong message for ministers to be spending millions of pounds of taxpayers' money on two new, official planes at a time when jobs are being cut in the NHS
Chris Grayling
Shadow transport secretary
The long-haul plane, which is likely to be wide-bodied, with a separate VIP area and kitted out for modern communications, will be used in place of the British Airways 777 currently chartered by Downing Street.
Shadow transport secretary Chris Grayling told BBC News 24 it was "the wrong moment to be splashing out taxpayers' money on funding the government to travel in style".
He added that if he became transport secretary, he would use Eurostar to get to places like Brussels, rather than take a plane.
Anger over US planes
Liberal Democrat leader Sir Menzies Campbell questioned whether the aircraft represented "good value for the British taxpayer".
"We are entitled to ask what the prime minister intends to do to reduce the environmental impact of his travel," he said.
But Labour replied that all Mr Blair's flights had been "carbon neutral" since April 2005.
There was also criticism over speculation that at least one of the aircraft is likely to be from US-built Boeing.
A spokesman for the Transport and General Workers Union, said it was "extraordinary that the prime minister should not be flying in a British-built plane".
"This sends a further negative message about the future of manufacturing."
Call sign
A spokesman for the GMB union added: "GMB members who work in the aircraft industry will be dismayed that the government is preferring to buy an American aircraft rather than one built by a UK manufacturer."
US presidents have had their own planes since 1944 when Franklin Roosevelt started using the "Sacred Cow".
In 1962, the first jet aircraft, a Boeing 707, was purchased for use and President Bush now uses two specifically-configured Boeing 747-200B planes.
They are commonly known as "Air Force One" because that is the call sign used when he is on board either plane, or any other Air Force aircraft.
UK Prime Minister Tony Blair is set to give the go-ahead for an order of two government jets. One jet is likely to contain 70 seats, whilst the other will be smaller with around 15 seats. Previously the Prime Minister has chartered aircraft or used a royal plane. Both planes would be second hand and not bought outright but leased on a long-term contract.

Government departments, as well as the royal family, who will have first priority, will be able to use the jets, but will have to pay for the full cost of travel. 

The story was first reported by BBC correspondent James Hardy, and comes out of a study into ministerial travel costs.

The nicknames 'Blair Force One' and 'Blair Force Two' have already begun to stick. American presidents have traditionally used a special jet called 'Air Force One'.