The union is claiming for a flat rate of£4,000 to be paid to all local government workers. It seeks to replace the current rates, which range from£1,407 in outer London to£2,674 in inner London.
Local government employers have rejected the claim. The first day of strike action will go ahead on 14 May, unless the employers return to the negotiating table.
Unison's chair of local government in London Dave Eggmore said: 'With the police now paying up to£6,000 in London allowances and many inner London nurses getting over£4,000, it is time local government workers caught up. Our members' claim is not excessive and a flat rate would help the poorly paid the most.'