The Electoral Commission announced its decision after a long-drawn investigation into the party's reported spending on three by-elections in 2014 and the 2015 General Election, led and won by former Prime Minister David Cameron.
The fine was imposed after the commission concluded that the Tory party had failed to record or incorrectly recorded payments totalling more than 250,000 pounds.
This is the biggest fine imposed on a political party by the watchdog.
The rules established by parliament for political parties and their finances are there to ensure transparency and accountability," said John Holmes, chair of the Electoral Commission.
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"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 said.
(Reopens FGN 19)
The Commission has also concluded that Simon Day, the
registered treasurer of the Conservative party until April 2016, committed three contraventions under section 41 and two offences under section 82(4)(b) of Political Parties, Elections and Referendums Act 2000.
The allegations that prompted the investigation were that the Tories bussed in activists to take part in critical campaigns in marginal seats which helped Cameron win a majority at the 2015 General Election.
The Conservative party has blamed an administrative error and said it had complied with the investigations and will pay the fine fully.
A party spokesperson said: "CCHQ (Conservative Campaign Headquarters) accepted in March 2016 that it had made an administrative error by not declaring a small amount constituting 0.6 per cent of our national spending in the 2015 election campaign.
We regret that and will continue to keep our internal processes under review to ensure this does not happen again.
"It also called for a "review" of how the Electoral Commission's processes and requirements could be clarified or improved.