The Liberal Democrats have been hit with a major fine for failing to declare nearly £185,000 of spending in the last general election.

The party was told to pay the maximum £20,000 penalty by the election watchdog, while a criminal investigation has also been launched into payments missing from the party’s returns.

The Electoral Commission found invoices supporting 122 of the payments were completely missing from the party’s documentation and “evidence indicating the party was aware of some of the missing payments”.

Bob Posner, director of political finance and regulation and legal counsel at the watchdog, said: “Our investigation uncovered systemic failures in ensuring that the rules were being followed.

“The party and its officers co-operated fully throughout the investigation. However, this is an experienced party that failed to meet the basic requirements of the law, and cases like this undermine voters’ confidence in our political finance system. This is why we have applied the highest financial penalty available to us.

“This also highlights why we have been calling on the UK Government to make higher sanctioning powers available to us.

“With millions of pounds being spent by large parties looking to form national Governments, a fine of £20,000 is no longer a strong enough deterrent to ensure the rules are properly followed.”

The Electoral Commission launched their investigation in January after identifying “apparent discrepancies between the Liberal Democrats’ national party spending return and some candidate returns”.

A Liberal Democrat spokesperson said: “We always endeavour to ensure our reports of national campaign expenses are completed in full, in good time and according to all applicable rules.