Suggestions of a Tube strike on the day of the royal wedding have been condemned as "cynical" by Downing Street.
The criticism came after Aslef floated the idea of calling members out on April 29, when Prince William is due to marry Kate Middleton.
Drivers went on strike on Boxing Day in the dispute about bank holiday pay, which remains deadlocked.
London Underground took legal action to try to stop the Boxing Day walkout but failed and the strike went ahead, disrupting one of the busiest shopping days of the year.
Aslef is demanding three days' pay and a day off in lieu for working bank holidays.
Prime Minister David Cameron's spokesman rounded on the suggestion of a strike on April 29. "Any such plans will be seen as a cynical attempt to undermine a day of national celebration," he said.
Posting on social networking site Twitter, London Mayor Boris Johnson wrote: "Can't believe this. Surely not!"
London Underground managing director Mike Brown said: "Tube drivers already have very fair levels of pay and get around seven weeks holiday a year, including allowance for bank holidays. The Aslef leadership is tearing up long-standing agreements that cover pay and annual leave and demanding even more; triple time and another day off.
"Londoners will simply be stunned at such a claim. The only way to resolve this issue is by getting round the table for constructive talks. I urge the Aslef leadership to end pointless strike threats and work with us to resolve this without any disruption to London."
Aslef's general secretary Keith Norman said the union's executive had yet to formally discuss timing a strike to coincide with the Prince's wedding, and criticism was "premature", he said.