The leader of the rail drivers’ union has called Grant Shapps a liar over allegations the union was pushing an “unofficial strike” relating to overtime shifts.

The transport secretary tweeted that passengers using Avanti West Coast services should expect disruption on Sunday because of unofficial strikes. Aslef said that it had not pushed any industrial action on Sunday and said the shortage of services was because of the shortage of drivers.

Shapps tweeted: “UNOFFICIAL STRIKES: Passengers using Avanti West services should expect disruption today. Archaic rules from 1919 mean working on rest days is voluntary. Unions now stopping drivers volunteering – causing misery for public & staff who won’t get paid. We MUST modernise rail.”

Mick Whelan, general secretary of Aslef, replied to the tweet, saying: “Grant Shapps is a liar. He is not only wrong, he knows he is wrong.

“His propensity to lie is just one reason why he could not get enough backers to get in the ballot to become Tory party leader. That and the fact that he is not very good at his job.

“The truth is that the company does not employ enough drivers to deliver the services it promises passengers it will run. The company, and the minister, should stop lying and start recruiting.”

Avanti West Coast said it had previously always had staff volunteer for overtime in order to cover weekend shifts for about 250 trains – with a “pool of extremely reliable drivers” willing to do so. “We have a longstanding agreement with drivers for them to work shifts as overtime,” a spokesperson said.

“Literally overnight, the number for this week has dropped to fewer than 10 trains covered by overtime. The drivers have, en masse, withdrawn their volunteering.”

The spokesperson for the firm said it was now facing “multiple short-notice cancellations” on its network from Monday and that cancellations were likely to continue until the current industrial dispute is settled.

“We are sorry for the enormous frustration and inconvenience this will cause our customers and condemn the drivers’ actions. We urge them to engage in meaningful talks around modernising working practices and developing a railway fit for the 21st century,” they said.

More strikes are planned in August by Aslef, the RMT union and the TSSA in worsening rows over pay, jobs and conditions.

Aslef members at Avanti will strike on 13 August as part of the long-running dispute over pay and conditions, but the union said no action is being held before then.

Whelan has previously accused Shapps of lying about negotiations over this summer’s rail strikes after Shapps claimed the RMT is “stalling on reform” and that Aslef “is dragging its feet in negotiations”.

Whelan said Saturday’s strikes had been solidly observed and showed the strength of support. “That shows the solidarity of our members and their determination not to be pushed around by Grant Shapps, the Department for Transport, and the train companies.

“After keeping Britain moving during the pandemic, they expect our members, who have not had an increase in pay since 2019, to keep working, effectively for a pay cut.”