Westminster's Commissioner for Standards, John Lyon, has launched a preliminary investigation after accepting that the spending "may not have been in accordance with the Code of Conduct for Members of Parliament".

Mr Lyon decided further scrutiny was needed after receiving a complaint from pressure group the TaxPayers' Alliance in February.

In a letter to the TaxPayers' Alliance, Mr Lyon wrote: "In essence, the part of your complaint I have accepted is that Mrs Martin's expenditure on taxis may not have been in accordance with the Code of Conduct for Members of Parliament and its associated rules."

He added: "I am inviting Mr Speaker to let me have his comments on your complaint. Once I receive his response, I shall consider how to proceed."

A spokeswoman for the Commissioner's office confirmed that a preliminary investigation had begun.

News of the probe will heap more pressure on Mr Martin, who has been repeatedly criticised over his own use of public funds.

At the weekend it emerged that more than £700,000 has been spent improving his grace-and-favour residence in Westminster since 2001.

Meanwhile, questions have been raised over why he has claimed more than £75,000 in MPs' allowance for a second home, despite having no mortgage on the Glasgow property.

Mr Martin has also come under fire for his determination to block the publication of details of the generous allowances given to MPs.