GlaxoSmithKline has hailed a major step forward for an ovarian cancer drug it acquired last year, announcing that it has been effective in stopping the spread of ovarian cancer.

The announcement is a significant boost for the drugs giant, which has come under pressure from investors who are concerned that it is failing to keep pace with some of its industry peers.

GSK said Zejula demonstrated its effectiveness in stopping the spread of the cancer when taken as the first treatment after a patient has undergone chemotherapy.

It gained Zejula in December as part of its controversial 5.1 billion US dollar (£4 billion) acquisition of oncology-focused company Tesaro.

When the acquisition was announced, investors flagged their concern over the entrance into a particularly competitive area of the drugs market, with shares in GSK immediately sliding 7%.

GSK chief executive Emma Walmsley is likely to see the positive testing of Zejula as a vindication of the acquisition.

Zejula is a PARP inhibitor drug - a promising type of cancer drug which works by blocking enzymes involved in repairing damaged DNA, thereby helping to kill cancer cells.

Investors have previously criticised GSK because the drug has lagged behind AstraZeneca-backed rival Lynparza, which has significantly higher sales.

GSK now appears more optimistic about the drug, after saying that it has now "met its endpoint of a statistically significant improvement in progression free survival for women".

Dr Hal Barron, chief scientific officer and president of research and development at GSK, said: "Almost 300,000 women around the world are diagnosed with ovarian cancer every year, yet only about 15% of patients are currently eligible to receive PARP inhibitors as their initial therapy.

"These exciting data demonstrate that Zejula has the potential to significantly benefit even more women with this devastating cancer."