Andrea Leadsom has held properties in a company rather than in her own name and made use of offshore banking services

The new Tory Treasury minister Andrea Leadsom has insisted she received no tax advantages from making use of offshore banking services, holding properties through a company rather than in her name and creating trusts for her children.

The moves are legal but Labour has raised "serious questions" about Leadsom's tax arrangements, calling on her to explain whether she had a lower liability as a result, especially given George Osborne's suggestion that complicated avoidance measures are morally unacceptable.

The controversy arose after it emerged that Leadsom, a former Barclays banker, created a property company called Bandal with her husband in 2003 to take ownership of two buy-to-let properties.

This is a structure that can have tax advantages because it means the owner pays lower-rate corporation tax, rather than income tax, on earnings from the properties, although she would also at some point have to pay tax on any dividends drawn.

Some of the shares in the company were registered to children's settlement trusts in 2005, a year before a crackdown by Gordon Brown that meant any new assets put into trusts from then on would be subject to inheritance tax or high charges.

A debt charge on Bandal was registered to Jersey-based Kleinwort Benson (Channel Islands) Ltd, an offshore banking centre, according to documents from 2006. This appears to have been switched to the London branch of the bank more recently.

Leadsom, who was elected MP for Northamptonshire South in 2010, stepped down as a director of the family company in February to be replaced by her 18-year-old son.

This was shortly before she was made economic secretary to the Treasury with responsibility for the multibillion-pound help-to-buy scheme, in a reshuffle prompted by the resignation of Maria Miller as culture secretary.

However, a spokesman for Leadsom insisted there "is not, nor has there been any tax advantage whatsoever" from the arrangements.

"This is a completely UK company that is eligible for UK tax on all profits. There is no offshore element and no tax avoidance," the spokesman said, while confirming that Leadsom has always been resident in the UK for tax purposes.

The Treasury had no comment to make.

In a speech this month, the chancellor, was highly critical of offshore banking. He has previously worked on international efforts to bring transparency to tax havens. "A very important part of our economic plan is that everyone makes a fair contribution. The message is very simple – if you're hiding your money offshore, we are coming to get you," Osborne said.

Shabana Mahmood, a shadow Treasury minister, said Leadsom "urgently needs to explain these arrangements and why they were put in place. Most importantly, she needs to explain whether she has benefited from a lower tax liability as a result.

"Families are £1,600 a year worse off since 2010 and the government's top-rate tax cut has seen millionaires £100,000 a year better off.

"The amount of uncollected tax has increased by £1bn this year and whilst George Osborne claims to be tackling tax avoidance, his deal with Switzerland raised less than a third the amount he originally claimed it would.

"So the public are rightly angry when the tax system is manipulated by those with the money to do so. Ministers responsible for this state of affairs must themselves be above board and seen to be above board, so it would seem there are serious questions to answer here."

Lord Oakeshott, a former Liberal Democrat Treasury spokesman, said he was surprised that Leadsom "had loans from banks in the Channel Islands while she was an MP" and added: "I can see why she would have wanted to clean all that up before becoming a Treasury minister."