More high-profile MEPs were being lined up to pledge their support for Joseph Muscat's bid for Labour's top job but the endorsements were cancelled after the negative reaction to that of Martin Schultz, The Times has learned.

On Friday Mr Shultz, the head of the Party of European Socialists (PES) in the European Parliament, made a surprise appearance at a press conference in Malta, pledging his support for Dr Muscat's leadership bid. He lauded the MEP's European credentials, saying that he was here to support "his MEP".

The move, however, sparked an angered reaction by the other contenders, who immediately wrote a strongly-worded letter to PES president Poul Nyrup Rasmussen, calling for the group to condemn and disassociate itself from the endorsement.

"This act, in our opinion, amounts to interference in our party's internal matters," said the letter, sent by Michael Falzon on behalf of the other contenders. "Not only so, but we are sure you would agree, that it is the Malta Labour Party as a whole which is a member of the PES, and not just one particular individual, and, therefore, we expect the PES to disassociate itself categorically and promptly from, and to condemn Mr Schulz's actions."

Contacted yesterday, Dr Muscat confirmed he had called off endorsements by a number of MEPs as a result of the reaction to Mr Schultz's endorsement.

He would not name the MEPs but said that given the "malaise" that the endorsement had created within the party, he decided to call off similar pledges that were being planned.

He qualified that this didn't mean he agreed with the position taken by the other candidates, insisting that such endorsements are common practice in democratic countries.

Nonetheless, he said he would rather not further the acrimony.

Fielding questions during an activity on Sunday, he said that he took note of what the other contenders had said on the matter and he would now let the delegates decide.