Channel 4 News has seen a cache of secret emails which suggest the prime minister’s chief of staff, Nick Timothy, may have helped the Conservative Party break electoral spending rules in South Thanet during the general election in 2015.

Tory candidate Craig Mackinlay saw off a strong challenge from former UKIP leader Nigel Farage in South Thanet. But the seat is now one of 29 where the Tories are under police investigation following allegations the party did not properly declare hundreds of thousands of pounds in campaign spending.

The legal spending limit for the South Thanet campaign was a little over £15,000. The Tories spent a further £14,000 housing Nick Timothy, who had been a special adviser for Theresa May at the Home Office, at the Royal Harbour Hotel in Ramsgate, along with several other Tories. But the party insisted they were working on the national campaign, not the local campaign.

The emails seen by Channel 4 News appear to cast serious doubt on this assertion, with Timothy appearing to be deeply involved with the South Thanet campaign.

For example, on 29 March 2015 an email sent at 3:58 p.m. asked: “Are we not putting ‘two horse’ race on everything? Don’t we need to?”

At 5:38 p.m., Timothy sent another email headed “First draft of messages”, containing a detailed, 500-word “message sheet” including slogans such as “Craig Mackinlay is a local man through and through”.

“Let me know what you think of the first cut of the message sheet,” he wrote. “So far, I’ve only worked off the polling book, some local research, and what Craig seems to have been saying locally.”

Timothy underlines that “We have to define this as a two-horse race between us and UKIP as soon as possible”.

Shortly afterwards, leaflets headed “IT COULDN’T BE CLOSER! Election in South Thanet is a Two-Horse Race Between the Conservatives and UKIP” were distributed throughout South Thanet.

Channel 4 reports the leaflets contain phrases matching some of Timothy’s messaging word-for-word and bear an imprint stating they were promoted by Mackinlay’s local agent.

There is also some suggestion that Timothy may have breached the Civil Service Code of Conduct by participating in party political campaigning whilst on the government payroll. But Civil Service chief Sir Jeremy Heywood has stated he will not be mounting an investigation.

In November 2016, the Conservative Party issued a statement officially denying Timothy had played any role Mackinlay’s campaign.

“During the GE 2015, Mr Timothy was a volunteer for the Conservative Party,” it read. “His role included briefing policy and political work on Home Office policy, briefing party spokespeople on Home Office policy, supporting Theresa May, and working on a variety of other matters for the Conservative Party during the campaign.”

As investigations by the police and the Electoral Commission continue, the question may become a matter for the prime minister herself.