"Other campaign items were inaccurately recorded in the long campaign rather than the short campaign, where spending limits are lower; and UKIP ignored Electoral Commission guidelines in dividing costs, leading to Farage sharing equally campaign costs with 65 local district and parish council candidates".
The watchdog highlighted the role played by a team of national officials based in the constituency during the 2015 general election, including a senior campaigns officer, a senior press adviser and two political advisers.
The Conservatives argued that they had, unusually, based their national anti-Ukip campaign in the Kent constituency, mirroring the fact that Ukip's national campaign was also there.
In a statement McCartney said: "The Conservative Party advised us that the so-called campaign "battle buses" were, as at previous general elections and in keeping with the practice of both the Labour Party and Liberal Democrats, a national campaign expense".
On Sunday Mr Farage said he would "probably" stand again in any future Thanet South by-election triggered by the alleged scandal.
"Consequently it appears that the party understated the spending it incurred on Mr Mackinlay's campaign, and as a result there is doubt as to the accuracy and completeness of his election expenses return".
CCHQ has always taken the view that its nationally directed battlebus campaign - a highly-publicised and visible activity with national branding - was part of its national return, and it would have no reason not to declare it as such, given that the Party was £2 million below the national spending threshold.
He said: "Politics is not a game".
The disclosure follows the revelation that Craig Mackinlay, who defeated Ukip leader Nigel Farage to hold Thanet South for the Tories, was questioned for six hours last week by Kent Police.
The Met statement said: "The Metropolitan Police received complaints or referrals that resulted in four allegations of possible irregularities in relation to expenses returns for 2015 General Election".
"Therefore it would not be appropriate to comment further".
"Officers from Kent Police continue to work with the Electoral Commission as the investigation continues".
DS Steve Maxfield of Staffordshire Police's fraud and financial investigation unit said: 'A file has been submitted to the Crown Prosecution Service'.
Mr Mackinlay, and his office, declined to comment when contacted by KoS throughout the week.
Police forces from across the Midlands are among those who have passed files to prosecutors as part of a probe into the Conservatives' 2015 election expenses.
Farage has claimed that the Tories "cheated" in the race and "swindled" the voters.
The Electoral Commission is also looking into the allegations which followed a Channel 4 News investigation.