The Conservatives are facing claims that their decision to use a primary process to select a candidate for the Rochester and Strood by-election could mean they exceed campaign spending limits.

The party is using a postal primary to select its candidate and is due to confirm whether it will be barrister Anna Firth or marine surveyor and Medway councillor Kelly Tolhurst later this week.

The possibility that in doing so, the party has already incurred election expenses ahead of the formal campaign, has been made by the Channel 4 political reporter Michael Crick.

In his blog, he says expert election lawyers have told him there could be a case that selecting a candidate using a primary process, in which every voter is able to vote regardless of whether they are a party member, might count as formal election expenses.

He writes: “Both believe that the primary process could open the Conservative contender to an election petition in the event of them winning.”

Parties are allowed to spend up to £100,000 on campaigns, with the law stating the limit applies during the “regulated period.” This begins “on the day after the date you formally become a candidate and ends on the date of the poll”.

The Conservatives have already rejected similar claims made by Ukip, saying the spending limit only applies once a candidate has been confirmed.

The Tories have already mailed literature to residents three times, chiefly to publicise the use of a postal primary to select its candidate.This is thought to have cost thousands of pounds.

A Conservative spokesman said: ?"When we have a candidate they will submit a return of election expenses in accordance with the law."