Thousands of postal workers took part in an unofficial strike in London on Wednesday.
Union leaders have threatened to disrupt the Christmas mail
The Royal Mail said it believed less than 2,000 employees joined in the industrial action over a series of disputes, including the suspension of union officials.
About 1,000 of the workers who took part are believed to be at Greenford in west London.
Two official strikes by the Communication Workers Union (CWU) have already gone ahead in recent weeks in a dispute over London Weighting allowances.
The postal workers want London Weighting increased to £4,000, but Royal Mail has said there is no more money to improve an offer of £300 a year more, taking inner London allowances to £3,784 and the outer London figure to £2,667.
But union leaders have threatened to disrupt the Christmas mail if a solution cannot be found.
Further industrial action could go ahead in north London where hundreds of postal workers are due to vote over supporting a colleague who claims he was denied a transfer to the Midlands.
Tom Doherty, who was sacked after he was accused of being a football hooligan, was reinstated after winning his case of unfair dismissal.
A spokeswoman for Royal Mail said it was "nonsense" to suggest that anyone was trying to block Mr Doherty's transfer.
"We are committed to supporting Tom in his transfer as soon as we can."