someone claiming to have been a candidate at the election, or
someone claiming to have had a right to be elected or returned at the election, or
an elector (not an anonymously registered elector) who had a right to vote at the election (although they need not have voted)
The allowable grounds for a petition are that there has been an:2
undue election, or
undue return
There is a separate judicial process for challenging the election of an MP on the grounds that they were or are disqualified under the House of Commons Disqualification Act 1975 (as amended). In that case, an application may be made to the Privy Council for a declaration to that effect (provided that a petition is not pending or an Order of the House of Commons to disregard the disqualification has not been made).
The Member whose election or return is complained about must be a respondent to the petition.3
If the petition complains about the conduct of the (Acting) Returning Officer or their staff during the election, the (Acting) Returning Officer must also be a respondent.4
Normally, a petition must be presented within 21 calendar days after the date of the return of the writ5
(which in most cases will be the day after the election) and can be lodged at any time up to, but no later than, 12 midnight on the last day. However, if the petition complains of corrupt or illegal practices involving the payment of money or other reward, or an illegal practice relating to election spending, further time may be allowed.
For any questions relating to election petitions, including to confirm the deadlines for lodging an election petition, you should contact:
In England and Wales:
The Election Petitions Office Room E105 Royal Courts of Justice Strand London WC2A 2LL
There are costs attached to an election petition. If you are considering lodging an election petition, we strongly recommend that you take independent legal advice.