Lawyers claiming that Vote Leave broke electoral law during the 2016 EU referendum campaign have been helped by anonymous donors.

A 50-page dossier published yesterday prompted calls for an “urgent investigation” to establish whether a prosecution could be brought over allegations that its campaign broke spending limits. The document and legal opinion from QCs was released by Bindmans, a law firm acting for Christopher Wylie, a former director of research at Cambridge Analytica, the data company, and Shahmir Sanni, a former Vote Leave activist.

Tamsin Allen, a partner at Bindmans, said that donors had to be sought to support their work for Mr Wylie, who had been talking to the law firm for months about Brexit and the behaviour of Cambridge Analytica, which was hired…