A senior Tory backbench MP has accused HMV bosses of committing theft by continuing to sell vouchers when they must have been aware "there was little prospect of those vouchers or gift cards ever being redeemed".

Sir Tony Baldry, a practising barrister specialising in commercial law, said "directors and management must have known that the company was at very real risk of failure" whilst they continued selling vouchers "all through Christmas and up until the day they went into administration".

His comments came as voucher experts said consumers have probably lost at least £100m in now worthless HMV vouchers. Andrew Johnson, the director general of the UK Gift Card and Voucher Association, said many retailers sell up to 60% of the £4bn of vouchers sold in the UK every year in the run-up to Christmas.

Baldry, MP for Banbury, said there was a "legitimate question" of whether HMV's directors were "obtaining property by deception, ie offences against the Theft Act, in allowing their stores to continue to sell vouchers and gift cards when they must have known that there was little prospect of those vouchers or gift cards ever being redeemed".

Just days before Christmas, HMV's bosses warned investors that there was "significant doubt on the group's ability to continue as a going concern in the future". Yet it continued to sell vouchers to the public, many of whom were unaware of HMV's parlous financial position, until just hours before the company entered into administration.

A spokesman for HMV's directors said: "Until as late as early afternoon on Monday, the directors believed that they had a reasonable prospect of avoiding insolvency and were satisfied that they were complying with all of their legal obligations including in respect of gift cards. When it became clear to the board late on Monday afternoon that they had no option than to file for administration, they issued immediate instructions to all stores to stop selling gift cards."

HMV and Deloitte, its administrator, refuse to say how much money consumers have lost on vouchers they are refusing to honour. Other administrators said Deloitte must know how much money is held in unsold vouchers.

Baldry said in a letter to the Department for Business (BIS) it was "more than unfair" that consumers should be left out of pocket when retailers refuse gift vouchers. He called on BIS to change the law to protect the public. "A gift voucher should be as good as a bankers draft, ie a consumer should know that they will either be able to redeem the gift voucher, or get their money back, and there is absolutely no reason why companies shouldn't keep monies raised from gift cards or vouchers in a separate account," he said.

Ian Murray, Labour's shadow minister for consumer affairs, said he had written to HMV and Deloitte to ask them to immediately disclose how much money the public has lost in HMV vouchers. "It will strike consumers as unfair that whilst the company is still trading, they are unable to use gift cards and vouchers," he said. "I hope that HMV and its administrators will be able to reconsider their decision."

Johnson said the board of the GCVA would discuss whether to allow a resurrected HMV to remain a member of the association if it continues to refuse to honour the vouchers. He added that public pressure could force the administrators to reverse their decision and honour the vouchers. The same team of Deloitte administrators started accepting Comet vouchers following publicity about a four-year-old cerebral palsy sufferer unable to buy an iPad with vouchers from the electricals retailer that collapsed before Christmas.

Richard Lloyd, executive director of Which?, said it was "outrageous" that consumers have been left out of pocket. "We want the rules on gift vouchers and insolvency to be reviewed to ensure consumers are adequately protected in cases like this," he added.

Dean Dunham, founder of consumer rights website youandyourrights.co.uk, said it was "morally appalling" that HMV's bosses continued to sell vouchers "when they would've known they were in trouble and going into administration, and they knew the first thing administrator would do would be to take them."

HMV Ireland collapsed into receivership yesterday and 16 shops in the Republic closed their doors. Receivership is more serious than the administration process in the UK, which increases fears for HMV's 300 Irish jobs.

A grandfather who bought his grandson a €40 HMV gift voucher for Christmas walked out of a Dublin HMV with three computer games on Tuesday after staff refused to accept the voucher.