Sports Direct has extended its bet on Debenhams by a year and reiterated that it would be a supportive shareholder in the department store chain.

The sportswear retailer, controlled by the billionaire Newcastle United owner Mike Ashley, agreed a so-called put-option deal with Goldman Sachs over a 10.5% stake in Debenhams in January 2015. On Thursday morning the company said the agreement had been extended by one year.

The put-option is a bet that Debenhams’ share price will rally. If the share price is below the agreed exercise price when the contract expires, Sports Direct would have to buy the stake – the equivalent of 128.9m shares – at that price or pay the bank the difference. If the share price rises above the exercise price, Sports Direct receives the premium – a profit for Ashley and the company.

Sports Direct previously said its maximum exposure under the put-option was about £85m. Buying put-options is a way of gaining access to a sizeable stake in a company without tying up capital.

The retailer said: “Sports Direct reiterates its intention to be a supportive stakeholder in Debenhams and to create value in the interests of both Sports Direct’s and Debenhams’ shareholders.”

Debenhams’ share price rose slightly to 74.4p, from 73.35p on 23 January 2015 when the put-option deal was first announced.

The agreement is Sports Direct’s only remaining put-option that relates to shares in other listed companies, it said. The group had set up a similar deal over Tesco shares.