Finland's police announced on Friday that they will investigate whether the prime minister's breakfasts have been illegally subsidised using taxpayers' money.

Prime Minister Sanna Marin found herself in hot water on Tuesday when the tabloid Iltalehti reported that she has been claiming back about 300 euros ($365) per month for her family's breakfasts while living at her official residence, Kesaranta.

While opposition figures accused the PM of being left with egg on her face, the 35-year-old head of government insisted that the perk was also given to her predecessors.

"As prime minister I have not asked for this benefit nor been involved in deciding on it," Marin said on Twitter.

Legal experts consulted by the media subsequently suggested that using taxpayers' money to pay for the prime minister's morning meal may in fact contravene Finnish legislation.

On Friday, police announced a pre-trial investigation into a possible public-office offence, after receiving a request to probe the issue.

"The prime minister has been reimbursed for some meals, even though the wording of the law on ministerial remuneration does not appear to permit this," police said in a statement.

In the statement, detective superintendent Teemu Jokinen said the investigation will focus on the decisions of officials inside the prime minister's office, and "in no way relates to the prime minister or her official activities."

Marin said Friday on Twitter that she welcomes the investigation and will cease claiming the benefit while it is looked into.

The Social Democrat politician has enjoyed relatively high levels of public support since coming to office in December 2019, and her centre-left coalition has been credited with helping Finland maintain some of Europe's lowest coronavirus infection rates.

However, as the Nordic nation gears up for local elections on 13 June, her party lags in the polls behind the opposition, while the far-right Finns Party has been predicted to make record gains.