The way was cleared for Mr Patten yesterday when Downing Street announced that the Tory MP Sir Alastair Goodlad, the other main contender for the post, had instead been appointed High Commissioner to Australia.
Sir Alastair, MP for Eddisbury in Cheshire, was William Hague's nominee as the second British European commissioner to accompany Neil Kinnock in Brussels.
If he is approved by the European Commission, Mr Patten would succeed Leon Brittan, the vice-president whose term is due to end this year.
Tony Blair's decision to appoint Mr Patten instead of Sir Alastair was seen as a breach of convention and a snub to Mr Hague, but followed Foreign Office advice that Sir Alastair was not a sufficiently "heavyweight" candidate.
The Foreign Office confirmed yesterday that the former chief whip will take up his new post early next year, triggering a by-election. The Tories would have to defend a slim majority of 1,185.
Sir Alastair replaces Alex Allan, a former private secretary to John Major, who will return to a "senior Whitehall post", Downing Street said.
A spokesman said Mr Blair had a "high opinion" of Sir Alastair and believed he would do an excellent job at an important time for UK-Australia relations. Adding weight to speculation of a snub to Mr Hague, the spokesman said he would have been "informed rather than consulted" about the appointment.
Downing Street refused to be drawn on the issue of Mr Patten's appointment, but confirmed that the new commissioner would be revealed at the end of June.
Romano Prodi, president-designate of the Commission, is expected to announce the full list of his new team of commissioners, including Mr Patten, at the same time.
Mr Patten's nomination effectively ends the possibility of his ever leading the Tory party and as a result Mr Hague is understood to be pleased at the removal of a potential rival for his job. Michael Portillo, former defence secretary and darling of the Tory right, moved swiftly yesterday to rule himself out as a candidate in the Eddisbury by-election.
"Michael Portillo will not be putting his name forward for the by-election which will be caused by Alastair Goodlad's leaving the House of Commons," Mr Portillo's office said in a statement.
Under a long-held convention, each of the UK's two commissioners comes from the Labour and Tory parties, no matter who is in government at the time.
Mr Major undermined the convention when he vetoed the appointment of Mr Kinnock soon after the 1992 general election.
However, Mr Major was forced to change his mind two years later and the former Labour leader became transport commissioner.
Mr Kinnock has made clear his worries about Mr Patten's prospective appointment and has sought guarantees that he will remain the more senior of the two UK representatives.
As a result, Downing Street is believed to have lobbied for a vice-presidency post for Mr Kinnock in the new administration in Brussels.
On hearing the news of Sir Alastair's appointment, one Labour MP said: "We used to send convicts to Australia. Now we are sending failed Tory politicians."