A former writer for the Daily Mirror, at the 1945 general election he was elected to Parliament for the constituency of Gravesend in Kent.

In 1947, he wrote an article for the World's Press News alleging that members of parliament gave information to the newspapers about private parliamentary party meetings, often in return for money, publicity or free drinks. The allegation, which was considered a grave infringement of parliamentary privilege, was investigated by the Committee of Privileges, who decided there was no evidence to support them. The only exception was the case of Allighan himself and another Labour member, who were found to have sold such information to the London Evening Standard.

The other member, Evelyn Walkden, admitted the offence and since he had paid taxes on the money, was permitted to remain as an MP. Allighan was charged with "aggravated contempt and gross breach of privilege" and expelled from the House of Commons on 30 October 1947. In the debate, the Leader of the House, Herbert Morrison, proposed six months' suspension but it was argued that this would deprive his constituents of representation for too long a period. It was also observed that after expulsion he was free to seek re-election if he believed he had been treated unfairly and, if returned, could resume his seat. However, he chose not to do this and immediately resigned from the Labour Party. He appears to have been the only MP since Charles Bradlaugh in the 19th century to have been expelled from the Commons other than for a serious criminal offence or bankruptcy.

At the resulting Gravesend by-election, Richard Acland held the seat for Labour, with a reduced majority.