The Stratford upon Avon MP had been technically prevented from standing for Labour because of a rule which says candidates must have been party members for at least two years. But Tony Blair, the Labour leader, persuaded the NEC, by 15 votes to 5 to rule that Mr Howarth's situation was "exceptional" because, as a Labour MP, he is already a member of the Parliamentary Labour Party.
Mr Howarth is believed to be interested in a number of northern seats. Party members in Wentworth, South Yorkshire, have already expressed fears that the leadership intends to impose him as their candidate.
Yesterday's decision clears the way for Mr Howarth to be chosen in the few remaining ballots of local party members, but it also allows the leadership to "appoint" him as a candidate for a constituency if a general election is called before ballots can be organised.
A spokesman said the party had taken legal advice on its interpretation of the rules. The NEC decided in favour of permitting him to put his name forward because of his "right and principled stand" in switching from the Tories.
The NEC also voted narrowly to appoint a panel to interview candidates for Swindon North and effectively decide between Michael Wills, who won the original ballot, and Jim D'Avila, who claimed irregularities.Reuse content