15:37

Mark Serwotka, leader of the PCS civil service union, is to accuse Nicola Sturgeon of making vague promises to increase public sector pay, despite her repeated pledges to lift the 1% pay cap.

In a speech to trade unionists and political activists in the annual Jimmy Reid lecture in Glasgow on Thursday, Serwotka will say the first minister “cannot get off the hook” on public sector pay because Sturgeon has new tax powers and command over a significant public.

Held in honour of the Clydeside dockworkers leader, the lecture is organised by the pro-independence CommonWeal organisation, and Serwotka will urge pro-independence union members not to “end up inadvertently being soft on the Scottish National party” because that might damage the quest for a second referendum.

The PCS is launching its consultative ballot to test support for a UK-wide strike on public sector pay on Monday, during the SNP’s annual conference. The PCS is campaigning for a 5% pay rise across the public sector, a deal which the IFS think tank estimates will cost £7bn-£9bn a year.

It is campaigning on two fronts: the union has 8,000 members in devolved government jobs in Scotland whose pay is funded by the Scottish government, and another 16,000 in UK departments such as the MoD, HMRC and DFID whose pay will be agreed by the Treasury.

Serwotka told the Guardian: