Lloyds Banking Group has been hit with a legal challenge over pensions for its female staff.

The Lloyds Trade Union (LTU), which has 30,000 members, has instructed former head of pensions at Thompsons Solicitors, Ivan Walker, to launch a case before the Employment Tribunal.

Lloyds has not yet instructed an external firm, with in-house pensions counsel Giannis Waymouth leading the correspondence.

The claim will seek to “close the gap” in pensions for the bank’s female staff. The action relates to guaranteed minimum pensions (GMPs), which concern members of contracted out salary related pensions schemes.

A statement by the LTU said GMPs were “one of the last bastions of pensions discrimination” and the issue “needs to be resolved”.

The union estimates up to 148,000 women who currently or have previously worked at Lloyds have seen reductions in their pensions compared to their male colleagues.

Walker is understood to be bringing the claim for the LTU on behalf of 28,000 female members of the pension schemes. The case is at the letter before action stage, with Lloyds expected to respond before the end of August.

Walker is now principal of Walkers Solicitors, based in Kent. He has instructed Outer Temple Chambers’ Andrew Short QC on the dispute. Short is also currently acting on the headline equal pay case against Asda, brought by thousands of mostly female staff.

Lloyds Bank is currently facing a number of other large disputes. The bank instructed CMS Cameron McKenna to defend a multimillion-pound claim brought by Premier Motorauctions last July. It is also in the midst of a mammoth disclosure exercise in its court battle with 6,000 shareholders over its acquisition of Halifax Bank of Scotland (HBoS).

Lloyds cut back its in-house legal team for the second time in a year in March following a number of “reorganisations” of the team amid wider operational changes at the bank. The recent changes led to job losses for junior lawyers at its London headquarters.

Last year the bank also closed its regional litigation teams and made around 25 mid-level redundancies in favour of outsourcing its low-value disputes work to TLT and Eversheds.