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The former PM was seen on the outskirts of Kyiv on Sunday, as he comes under pressure over claims that the BBC chairman helped arrange talks which led to his £800,000 line of credit while at No 10.
Former Army chief Lord Dannett had previously told The Independent that Mr Johnson was a “loose cannon” whose plan could upstage Rishi Sunak, while ex-First Sea Lord warned the former PM against “looking for publicity”.
But Mr Johnson said he was invited by president Volodymyr Zelensky to visit the country once again – as he urged western leaders to give Ukraine “all the tools they need”.
In a statement on Sunday, the former prime minister said: “It is a privilege to visit Ukraine at the invitation of president Zelensky. The suffering of the people of Ukraine has gone on for too long.”
“The only way to end this war is for Ukraine to win – and to win as fast as possible,” he added. “This is the moment to double down and to give the Ukrainians all the tools they need to finish the job. The sooner Putin fails, the better for Ukraine and for the whole world.”
On Sunday Mr Johnson was seen making unannounced visits to two suburbs north of Kyiv – two areas where Russian troops are accused of atrocities after a failed push on the capital last year.
He is expected to meet Mr Zelensky later on Sunday, in what may be seen as a move to undermine his successor’s authority and bolster his own legacy of support for Ukraine.
Mr Johnson is understood not to have asked No 10 for permission for this visit – his fourth trip to Ukraine since the Russian invasion. However, Downing Street said Mr Sunak was “supportive” of Mr Johnson’s latest visit.
The prime minister’s press secretary said on Sunday that Mr Sunak is “always supportive of all colleagues showing that the UK is behind Ukraine and will continue to support them”.
Asked last week about reports of a planned trip to Ukraine by Mr Johnson, Lord Dannatt told The Independent: “Boris Johnson has been a loose cannon all his life”, adding: “I don’t think he can do any harm, as long as he’s not trying to upstage Rishi Sunak.”
Lord West, former First Sea Lord, said ex-prime ministers “shouldn’t be looking for publicity and kudos – they should be helping support the UK government and keeping in the background”.
Senior Tory MP Tobias Ellwood, chair of the defence select committee, also said Mr Johnson should “not interfere” with the official relations between the countries when the possibility of Mr Johnson’s trip first emerged.
It comes as Labour called for an investigation after claims that the BBC chairman Richard Sharp helped arrange for a guarantee on a credit line of £800,000 weeks before he was recommended for the job by the then-prime minister.
The party has written to Daniel Greenberg, the parliamentary commissioner for standards, following a report in the Sunday Times that Mr Sharp was involved in talks about helping Mr Johnson when he found himself in financial difficulty.
But a spokesperson for Mr Johnson dismissed the report as “rubbish” –insisting the former PM had not asked Mr Sharp for financial advice and saying his financial arrangements “have been properly declared”.
Of Mr Johnson’s private dinner with Mr Sharp and Sam Blyth, the distant relative of the ex-PM who acted as the guarantor, the spokesman said: “So what? Big deal.”
Mr Sharp told the Sunday Times: “There is not a conflict when I simply connected, at his request, Mr Blyth with the cabinet secretary and had no further involvement whatsoever.”
Labour’s shadow culture secretary Lucy Powell MP has also now written to the Commissioner for Public Appointments, William Shawcross, asking him to investigate the process for appointing Mr Sharp.
Meanwhile, foreign secretary James Cleverly has claimed he would like “nothing more” than to see Ukrainians armed with German-made Leopard 2 tanks.
Mr Cleverly was asked on the BBC Sunday with Laura Kuenssberg programme whether he was disappointed Germany did not authorise the release of its Leopard 2 battle tanks.
The foreign secretary refused to directly criticise Berlin, saying it is for “every sovereign government to decide how they are best able to support the Ukrainians as a member of Nato”.
However, he noted there is nothing he would like more than to see the Ukrainians equipped “with those most up-to-date armoured vehicles”.
His comments come after a pledging conference in Germany on Friday ended without a commitment by Western allies to send more battle tanks to Ukraine, despite a call from Mr Zelensky to speed up the delivery of military support.
So far among the Nato allies, only the UK has agreed to send tanks, in the form of 14 British Army Challenger 2s. There had been hopes that Germany would authorise the release of its battle tanks, which are potentially available in far greater numbers.
US defence secretary Lloyd Austin said officials were “pushing hard to meet Ukraine’s requirements for tanks and other armoured vehicles”.