The Vatican has banned a small group of breakaway traditionalist Catholics from ordaining more priests and deacons.
The Society of St Pius X (SSPX), which split from the Vatican in 1970, plans to ordain more than 30 men in June.
But in a statement, the Vatican said sect members were not entitled to exercise their ministry and that any ordinations would be "illegitimate".
In January this year, Pope Benedict XVI revoked the 21-year excommunication of four bishops in the Swiss-based group.
The decision provoked an angry reaction as one of the SSPX members, Bishop Richard Williamson, is a known Holocaust denier.
The Pope defended the move by saying it was important to ensure the unity of the Catholic Church.
But instead of returning to the Church as the Vatican had hoped, SSPX has announced the new ordinations, planned to take place in Switzerland, Germany and the US, says the BBC's David Willey in Rome.
The situation is embarrassing for the Vatican, with no easy way to prevent the Pope from losing face if - as seems likely - the ordinations go ahead, says our correspondent.
SSPX was founded by a French archbishop, Marcel Lefebvre, in 1970 as a protest against the Second Vatican Council's reforms on religious freedom and pluralism.
It claims to have almost 500 priests as members and says it is active in more than 60 countries.
