Boris Johnson has received another £15,000 from the pro-Brexit digger maker JCB, figures show, part of a mass of donations to potential Conservative leadership contenders with the expectation that Theresa May’s time in office is coming to an end.

The former foreign secretary, a likely standard bearer for pro-Brexit Tories, received £31,000 in donations in the past month, the register of MPs’ interests shows, and has been given almost £140,000 in money or other support since late last year.

Others to receive new donations in recent weeks include Dominic Raab, the former Brexit secretary who has made no secret of his leadership ambitions. He has been given more than £50,000 in cash and other donations this month alone.

In January, JCB – owned by Anthony Bamford, a Conservative peer and donor – gave Johnson £10,000 three days before he made a speech at the company’s Staffordshire headquarters, during which he repeatedly praised it.

As well as the new £15,000 from JCB, given in mid-February, Johnson accepted £16,000 from Johan Christofferson, who co-owns a New York-based investment business. He gave another £20,000 in January.

Johnson has been very publicly preparing for a new tilt at the leadership if May is ousted over the Brexit deadlock; some Conservative MPs predict that could happen as early as next week if her deal is rejected again.

Despite recent controversies, such as Johnson’s remarks last week that funding for inquiries into non-recent sexual abuse was money “spaffed up a wall”, he has recently tried to appear less chaotic – and less scruffy.

A backer of his campaign said it was a deliberate move for him to smarten up his image in an attempt to look more prime ministerial, with a haircut and weight loss.

Raab, who already has a semi-official leadership campaign running under the slogan Ready for Raab, was given a donation worth more than £44,000 to pay for a staff member for six months, from the London-based bank Arbuthnot. He received £10,000 from another, private donor.

Other prominent Conservatives to receive donations include Amber Rudd, the work and pensions secretary, who accepted £7,500 from a private equity executive, and Michael Gove, the environment secretary, who received £3,000 from a firm of management consultants.

The set of MPs’ interests also show the Conservatives have accepted more money from the wife of one of Vladimir Putin’s former ministers, who has been a regular and generous recent donor to the party.

Lubov Chernukhin, whose husband, Vladimir, is a former Russian deputy finance minister, donated £9,500 in cash and other offerings to the Conservative MP and party chair, Brandon Lewis, in February and March. She has given more than £600,000 to the Tories in recent years.