The Electoral Commission has announced it is to investigate Labour's repeated failure to declare its donations on time.

The watchdog branded the latest late submission "unacceptable", having previously warned the party to meet the quarterly deadlines.

Publishing the latest register of party political donations on Friday, the commission said it would be writing to the Labour leadership to ask why the records were still not being kept up-to-date.

The investigation will embarrass the party, which legislated to establish the commission in 2000 in a bid to bring transparency to the donations process.

Electoral Commission chairman Sam Younger said the party's performance was contributing to the public's mistrust in politics.

"Persistent late reporting of donations undermines the efforts being made to ensure greater transparency in the finances of political parties," he said.

"The commission is determined to ensure that parties are doing everything they can to meet their legal obligations.''

In a statement, the commission said it "regards the Labour Party's repeated failure to report donations on time as unacceptable".

"The commission has written to the treasurer of the Labour Party announcing its intention to begin an investigation into the party's procedures for recording and reporting donations in order to be satisfied that all reasonable steps have been taken and due diligence exercised to ensure compliance," it continued.

"In the light of that investigation the commission will decide what, if any, enforcement action might be appropriate."

The Labour Party has offered assurances that it will co-operate fully with the review.