After a thorough investigation into the party’s campaign spending, the Commission found that payments amounting to £104,765 were missing from the 2015 election returns; payments totalling £118,124 “were either not reported to the Commission or were incorrectly reported by the party”; and the party “did not include the required invoices or receipts for 81 payments to the value of £52,924”.

It also discovered that the Conservatives “failed to maintain records explaining the amounts it invoiced to candidates in three 2014 by-elections for work on their campaigns and therefore the accuracy of the amounts could not be verified”.

The Chairman of the Electoral Commission, Sir John Holmes, said: “Our investigation uncovered numerous failures by a large, well-resourced and experienced party to ensure that accurate records of spending were maintained and that all of the party’s spending was reported correctly.

“The rules established by Parliament for political parties and their finances are there to ensure transparency and accountability.

“Where the rules are not followed, it undermines voters’ confidence in our democratic processes, which is why political parties need to take their responsibilities under the legislation seriously.”

He continued:  “This is the third investigation we have recently concluded where the largest political parties have failed to report up to six figure sums following major elections, and have been fined as a result.

“There is a risk that some political parties might come to view the payment of these fines as a cost of doing business; the Commission therefore needs to be able to impose sanctions that are proportionate to the levels of spending now routinely handled by parties and campaigners.”

The Conservatives have been given until April 13 to pay the fine.

Prime Minster Theresa May said: “They have imposed a fine on the Conservative Party and the Conservative Party will be…paying that fine.”

The penalty comes in the wake of the Chancellor of the Exchequer (Finance Minister), Phillip Hammond’s scrapping plans to raise national insurance (NI) contributions for the self-employed, which experts say would lead to a deficit of £2 billion in the government’s coffers  over the next five years.

He had announced the hike in his Spring Budget on March 8, but a week later Mr Hammond had to backtrack after Conservative MPs accused him of reneging on a campaign promise that NI contributions for the self-employed would not be increased.