Labour has accused the London Oratory, the grant-maintained Catholic school attended by Tony Blair's 12-year-old son Euan, of political mischief-making following its threat to become fee-paying if the party wins power.
David Blunkett, the party's education spokesman, claimed headteacher John McIntosh had misrepresented Labour's policy on GM schools in a "piece of inept and clumsy politicking".
The row broke out after Mr McIntosh expressed fears that Labour's policies might alter the school's character. He was quoted as saying: "If that meant that the school as it is now were to be destroyed, we would need to look at a range of options, which could include becoming an independent school."
Such a move would put Mr Blair in an awkward position, and allow the Conservatives to renew their attacks on Labour's "hypocrisy" towards selective schools.
It has also encouraged speculation - equally unhelpful to Labour - about a return to the 1970s when several direct-grant schools went independent rather than become comprehensive.
Responding to Mr McIntosh's comments, Labour stressed its proposals for "foundation" schools would give parents the leading voice on governing bodies while ensuring a fair admissions system and "sensible" local accountability.
It also promised dialogue to agree a balance between the central role of local education authorities and the autonomy of GM schools.
Mr Blunkett added: "In circumstances where schools are queuing up to return to the state system, it is highly unlikely that a school taking pupils from the variety of social backgrounds present at the London Oratory would choose to take the opposite direction.
"We feel there has been a little bit of political mischief-making which has nothing to do with lifting standards or education policy, and everything to do with party politics."
If the Oratory did go independent in the event of a Labour government, Mr McIntosh would have to find enough full-fee-paying parents, as Labour has vowed to abolish the Assisted Places Scheme.
According to Sir Bob Balchin, chairman of the Grant Maintained Schools Foundation, at least two other GM schools are thinking along the same lines as the Oratory.
He said GM schools were "pretty angry" about Labour's policies on nominating governors, financial autonomy and interference from LEAs. "They are perfectly content to be GM schools so long as they can keep the freedom they have won, often with great difficulty.
"It would be the greatest possible pity if a fine school like the London Oratory were compelled to take only children from wealthy homes like the Blairs' when currently it has a good social mix."